Hello, been reading this site for awhile now, have some info that might be helpful.

So, if Pfizer & Moderna do something similar to other pharma company batch #'s I've seen then for Pfizer the first letter is likely the year and the second letter corresponds to the month. So "E" would be 2020 and the letters later in the alphabet would be later in the year, with "F" as 2021 and we see "A" "B" "C" likely corresponding to Jan/Feb/Mar.

Likewise for Moderna we can see the "20" vs "21" and the letter preceding likely goes to month as well (the "21A" batches have early letters, etc.). The "A" at the end of the Moderna batch # may just be a way to differentiate between drug substance and drug product batches.