Caveat: In the absence of detailed sources, what follows is what can be said about the Leyden temperature scale. This should be taken as speculation until proven otherwise -this article could be completely erroneous.

The Leyden temperature scale could plausibly have been introduced around 1894, when Heike Kamerlingh Onnes' cryogenic laboratory was established in Leyden, Netherlands. The symbol is probably °L.

The scale is supposed to be the kelvin scale shifted so that the boiling points of hydrogen and oxygen become zero and 70 respectively. As it turns out, 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. If one supposes that absolute zero is at -20.15 °L, the purported definition is satisfied and, as a bonus, the shift between the Leyden, Kelvin and Celsius scales is a whole number in each case.

Source: [1]