Generally, Carter used a 2 character code for the date code. This was in the format "m" "y"; where "m" was a letter representing the month (A for January, B for February, etc.) and "y" was a number representing the last digit in the year produced.
Some date codes from the mid 1950's through the mid 1960's used a three character code, in which case the code was in the format "m" "w" "y" where the "w" was a number representing the week within the month. Thus the A37 decoded to the third week of January 1957.
One has to use the proverbial "grain of salt" with the year (and the month, see below). The 964s was not produced prior to 1950, and not produced after 1965, so the 7 represents 1957, but most would not have access to the discontinuance dates.
As to the month, some years the letter "I" was used, and some years the letter "I" was omitted. In the years with "I"; "L" would represent December. In the years without "I"; "M" would represent December.
Your earlier comment about the SA, SB, SC, etc. is absolutely correct. Engineering changes would have the suffix changed to reflect a minor change. There should be no difference in the OP's A37 and H37 IF both were in factory virgin condition. Differences that may exist can be attributed to different aftermarket kits, or the ingenuity of Dr. Goodpliers (the evil twin of Mr. Goodwrench)!
Really, the date code for an aftermarket carb (the 964s is an aftermarket carb) is trivial as is represents only the date of production for that specific unit, nothing else. On O.E. carburetors for numbers matching cars, the date code IS important.
Jon.
