It is quite possible that the bolted-on brackets are replacements for worn out riveted-on brackets. If the rubber bushings on the top posts of shocks go bad, the top mounting plates could get worn so badly that they would have to be replaced (I had to do that on the 1954 1/2 ton I am restoring).

Here is information on the locations of transmission serial numbers.

C210 could be a date casting code:
C = March
21 = 21st
0 = 1950

591665 is a casting code for an SM420 4-speed (1947-1953) - is that what you have?