I don't know when RWS began to mark the compression tubes with month/year. (It's on the side near the end on the left.) Prior to that, some guns may have been marked on the butt, if plain wood, with the month/year or only two digits of the year. I think I've seen some of Mike Driskill's older 27's marked this way.
Diana isn't one of those companies with serial number records. This is especially true of most European companies from WWII, back. Quite a few had records destroyed.
Sometimes the best you can do is ballpark the age with production run dates.
