1953 was the transition year for the SM318 from the two lug synchronizer to the four lug synchronizer. They also made some change to the gears. The casting numbers are the same for the early and the late but the internals are different. You really need to disassemble your transmission to see what you have.
Deegs, Just to follow up with what "crenwelge" posted, The two different 2nd to 3rd gear synchro hubs are identified as GM# 605020 (1940-53 3 speed - two lug - 24 external spline) and GM# 37051122 (1953-54 3 speed - four lug - 24 external spline). Doing a little web surfing this morning I found
this listing with both parts potentially being available. You would need to call the number at the top of the page for availability and price, but a possible option if needed.
Once you get into your transmission and figure out what your needs are, let us know and we can continue to offer help and leads.
