here is information from Novak's website. Looks like the difference is a weep hole on the 52.

1947-1953:
No ribs on the case
Grooved worm on the input shaft instead of neoprene seal
Synchro rings all differ from each other. 1st-2nd synchro ring is bronze
Weep hole at 6 o'clock on the bearing retainer snout
Six digit casting number on the case, usually like "59xxxx" e.g., 591665.

1954-1963:
Same as previous but uses a neoprene input oil seal
No weep hole at 6 o'clock on the bearing retainer snout

1964-1967: Same as previous except
1st-2nd synchro ring is aluminum and 3rd-4th synchro rings are identical to each other
Seven digit casting number usually like "37xxxxx", "38xxxxx" or "39xxxxx"

1968-1981: Military Con-Diesel version. Features the strongest case and optimized internals. Usually equipped with unique top cover, but can be converted to standard top-shift control.