Don,

According to info listed in a GM Master Parts and Accessory Catalog I was able to look at here Rear Axle Identification , the stamped "AF" code on you pumpkin appears to match a "3.90" code for 1955.2-1957 3100 and 3200 models (1/2tons).

I added a screen shot below with an arrow to help point out the tree in the forest of data on this page.

There is also a very old (2006) thread on this topic that discusses the "3707340" casting code. You can read more here: ID this rear axle

I am not familiar enough with the date code "1 21" that follows the "AF", but others with more experience will surely chime in with more details. Dave's reply above gives the most definitive way to confirm your "date" for the ring and pinion set in the pumpkin.

Hope this helps answer your question??? thumbs_up
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~ Dan
1951 Chevy 3 window 3100
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