Neil,
I believe the 5 digit 1921s were built and matched to "1919" pre-Alphabet (refered to as "1923") commercial frames.
I have read that when there were two Lugers in a unit with the same last two digits, they often stamped the 3rd digit on the side plate to avoid a mis-match.
Thanks guys..
I've been dropping bye on occaision to see if you "Experts" are leading anyone astray. So far you seem to know what you're talking about.
Too danged cold to be out in the woods, so thought I would stop over and add to the confusion.
Ron