They didnt modify the frame numbers after decommission from service, it remained the same.
My understanding is that whenever the second letter is an "A" it means it is "Pubblica Amministrazione", i.e. a bike supplied to a governmental agency.
they were supplied in contractual purchase batches, so it's usually a the three letter code + two digit numbers, as opposed to civilian series which had 1-2 letters and 5 digit number.