Thanks for the response. Based on the link you sent, it appears the numbers I am talking about are the date of manufacture (most likely date of assembly).
The block in question is a D1OE-6015-AA. It carries a casting number of 1A21 (January 21, 1971). The number stamped in to that boss or ear where the water pump bolts on is 1B19E (Febrary 19, 1971 I'm thinking). But the extra digit, "E" is what is throwing me off.
I'm not doing any kind of a numbers matching restoration here. It's a 1972 Bronco with a date of manufacture in November 1971.
I'm just super crazy about tracking down any and all numbers I find in my projects so I don't run in to issues down the road.