The locomotives were built in two batches of 20 at Horwich Works. The first batch, turned out in 1899, were numbered 1400, 1392–9, 1401–4, 700, 702, 708, 711, 718, 735, 737; the second batch of 1902 were numbered 1405–24. Those numbered in the 700s took the numbers of older locomotives which had been withdrawn; the others were given numbers at the end of the L&YR list which were as yet unused. Under the LMS, they were allocated the numbers 10300–39 in order of construction, but several were withdrawn before these numbers could be applied.
No. 737, the last locomotive of the first batch, is believed by John Marshall to have been the first British superheated locomotive. The front tubeplate of the boiler was recessed, creating a cylindrical space into which the superheater was mounted. This consisted of a drum 3 ft 6 in (1,067 mm) long through which were a number of tubes for the hot gases to pass through. It offered only a low degree of superheat – 95 °F (35 °C) above the normal steam temperature. The last five locomotives of the 1902 batch, nos. 1420–4, were given the same apparatus when new, but it was removed from all six by 1917.