Neil, by the 60’s most grades of HSS had been developed (the landmarks being Mushet steel in 1866, T1 in 1907 & M2 in 1936).
M2 can be adequately treated by heating to 1040-1100?c in atmospheric conditions, followed with a combination of brine quench until equalised followed by air until cool.
M42 will also harden that way, but as it won’t reach its full potential (upto HRC70!!!) without salt or vacuum treatment at temperatures very close to it’s melting point, it would be pointless buying the higher grade to then heat treat it like that.
As I understand it the issue with all HSS is the need for controlled heating for good results… Initally in several stages followed by a rapid burst of energy to take it from ~900?c up to the final hardening temp of 1100-1200?c, and then multiple long tempering cycles in very a narrow temperature band… it would be hard and labour intensive to achieve that without building or buying specialist equipment.