RIO Models are intricately detailed diecast and plastic 1:43 scale models made in Cernobbio, Italy on Lake Como, by a tool and die company of the same name.
Rio started as a sideline business interest to a diecasting company in Cernobbio operated by the Tattarletti family starting about 1961 (Sinclair 1979, p. 388). The origin of the name is unclear.
RIO models first appear to have been imported into the United States by David Sinclair, a model enthusiast who brought many previously unknown European model brands to the U.S. in the 1960s and 1970s (Sinclair 1979, p. 386-387). RIO Models, along with R.A.M.I., Safir, Brumm, Dugu Miniautotoys, Lledo, Minialuxe and Cursor Models took off on the beginning made by Matchbox Models of Yesteryear to produce replicas of classic cars mainly appealing to older collectors (Rixon 2005, p. 11). Of these companies, only R.A.M.I. appears earlier than RIO. According to Edward Force, the first four RIO Models were made in 1961 - two 1906 Italas and two 1919 FIATs (Force 1992,263-264). These toys also may have been made under the Stampopolastica name and manufactured for Dugu Miniautotoys. Soon however, the company was producing its own vehicles (not as contract work), and thus RIO became an independent diecast name.
A few new models a year were sporadically produced until the company had a line of over 30 cars by 1972 and over 60 by 1978 (Sinclair's 1978). During this time, it seemed model production was more a side interest to the tool and die business, as only relatively few models were introduced through the 1970s. Traditionally, models were supplied in 'top-up' and 'top-down' versions each being painted in just one color.
RIO models were always 1:43 scale and models were ultra detailed with between 45 and 75 individual parts (Sinclair's 1974, 2-3). Models seem chosen for a combination of popularity, rarity, and unique design, traits consistently making Rio selections appealing. At first, most models were vehicles chosen from the European Brass era and from the 1920s. This favoritism toward earlier cars is seen in the standard RIO logo of two different automobile horns from the brass era; a fancy snake and more mundane trumpet.
Later, models of vehicles all the way up to the 1960s and 1970s were produced, such as the Citroen DS 19 and even a 1970s FIAT 124 Sedan. Still, there is a 'classic' sense to the RIO line - the unique cars of an earlier era. One also gets a sense that RIO was ahead of its time in offering models to collectors, and not children, long before such a practice became more common in the 1980s. In the mid-1970s, RIOs would cost between $15.00 and $20.00 when most other diecast 1:43 scale cars hovered in the $5.00 to $7.00 range.
RIO models, however, can be very brittle and many seemed made from a zamac alloy that perhaps was heavier on the lead than the zinc - drop a RIO and it will smash into pieces - they were definitely not made for children (drop a diecast Corgi Toys car and it will just bounce off of the floor). Rio zamac was also prone to zinc rot and the metal often tended to crack and deteriorate.
The mid-1970s also saw unstable supplies and many retailers stopped carrying RIOs when models were only delivered a few vehicles at a time (Sinclair 1979, p. 388; Sinclair's 1978). In 1977, David Sinclair, the main RIO importer to the United States, wrote that there were collectors who had been waiting on certain RIO models since 1964 and that he could have used 1,000 examples of each instead of the trickle he received (Sinclair's 1977; Levine 2009).
Focus on classic vehicles
A perusal of the online store 'Carmodel' shows that RIO makes at least 25 different models today. They seem more solid and the company more aligned to the mainstream model car business. Packaging is more in line with other collector / toy car manufacturers. Perhaps this is now RIO's main business - its other tool and die pursuits perhaps pushed aside.
One reason RIO modernized was due to competition with newer companies producing similar products. In the 1970s, German Cursor Models and Ziss and French Minialuxe, though often plastic, were similar to RIO in style and classic models selected. The kicker, though, was Brumm Models, which started in 1972. Brumm, another Italian firm making classic diecast vehicles in a nearby town, was reportedly started by Reno Tattarletti of the RIO family concern, along with a couple of business partners (Sinclair himself has written that it was a brother-in-law who started Brumm; see Force 1992, 106-107; Sinclair 1979, pp. 388–389; and Sinclair's 1978). Though likely unrelated to RIO, Eligor Models was another high production diecast company, started about 1980 in France, and focusing on cars from the 1930s to the 1960s.
Though RIO models had more parts and more detail, Brumm seems to have outdone RIO in many ways, producing more models that are sturdier and better marketed (Sinclair's 1980). Quality of Brumm Models was excellent, packaging well-executed even in the 1970s, and models were cheaper, which ostensibly pushed RIO to up standards and promotional know-how.
Since this time, RIO has greatly improved presentation, often producing cars in unique dioramas and in unique historical situations. Commonly, RIO will insert into the box a small brochure giving specifications and historical information about the car (Vitesse Models of Portugal has a similar practice). Even today, there is a feeling of uniqueness and irresistibility to RIO's models that other classic car model producers have difficulty emulating. Models today usually cost $50.00 to $60.00.
In 2011 RIO Models was acquired by M4 models of Pesaro, Italy, makers of the Best Model and Art Model lines.
Force, Edward. 1992. Classic Miniature Vehicles Made in Italy. Schiffer Publications.
Levine, R. F. 2009. David Sinclair in the Driver's Seat. Lake Erie LifeStyle, web published December 1. 
Rixon, Peter. 2005. Miller's Collecting Diecast Vehicles. London: Miller's, A Division of Mitchell Beazley.
Sinclair's Auto Miniatures. 1974. Exact Scale, magazine of information and sales from Sinclair's. Erie, Pennsylvania.
Sinclair's Auto Miniatures. 1977. Typewritten newsletter, flyer focusing on Cursor. August. Erie, Pennsylvania.
Sinclair, David. 1979. Scale 1:43, a Survey for Collectors, in Automobile Quarterly, Vol. 17, No. 4, pp. 384–399. Princeton, New Jersey: Automobile Quarterly, Inc. in Association with the Princeton Institute for Historic Research.
Sinclair's Auto Miniatures. 1978. Typewritten sales newsletter, June 1. Erie, Pennsylvania.
Sinclair's Auto Miniatures. 1980. Typewritten sales newsletter, December. Erie, Pennsylvania.
Sweetman, Chris. 2008. Dugu Bernardi 3.5 hp, 1896. Hobby Talk forum. See important references on this webpage. 
RIO models are featured on the website of their present parent company M4: M4 S.A.S.