The Leiden scale (°L) was used to calibrate low-temperature indirect measurements in the early twentieth century, by providing conventional values (in kelvins, then termed "degrees Kelvin") of heliumvapour pressure. It was used below -183°C, the starting point of the International Temperature scale in the 1930s (Awbery 1934).
It has been reported  that the scale is the kelvin scale shifted so that the boiling points of hydrogen and oxygen become zero and 70 respectively, but this is unlikely to be true. Oxygen under a standard atmosphere boils at a temperature in the 90.15 to 90.18 K range. For hydrogen, it depends on the molecular variety. The boiling point is 20.390 K for "normal" hydrogen (made up of 75% orthohydrogen and 25% parahydrogen) and 20.268 K for pure parahydrogen. Under the purported definition, absolute zero would lie at -20.15 °L.
« The 1955 Leiden scale13 was used to convert helium vapor pressures into temperatures [...] (13) H. van Dijk and M. Durieux, in Progress in Low Temperature Physics II, edited by C. J. Gorter (North-Holland Publishing Company, Amsterdam, 1957), p. 461. In the region of calibration the 1955 Leiden scale, TL55, differs from the Clement scale, T55E, by less than 0.004 deg. » (emphasis added)
« It should be mentioned that below -183°C, the Leiden workers do not entirely agree with some of the other cryogenic laboratories, but use a scale of their own, generally known as the Leiden scale. » (emphasis added)