My first guess is a SM420 from 1951 based on the web.
Casting # 591907 is linked to several sources to the SM420. One source lists the same casting number for both 3 and 4 speed transmissions which seems unlikely but I guess anything is possible.
L181 looks like a date code. December 18 for a year ending in 1 (1951, 1961 etc. and based on a Stovebolt post more likely 1951). The same Stovebolt post has a link to a brochure on rebuilding the transmission.
http://www.stovebolt.com/ubbthreads/...51932&page=all