I have on my files a picture of the bottom of the Su-24 that shows a similar stamp to the one on ramseyd's An-12. The only difference is that it has also a large (about 1inch high) number 5 (decal) on the bottom of the rear fuselage. This #5 may correspond to the last digit of the stock number, which is 6910-12-306-3025. To verify this I would have to look at the other models but the sets are in a somewhat inaccesible part of my garage

and that will have to wait!
While I was looking at my files I noticed that the picture of the identification decal on the container says "Behälter 1 von 2" (the "von 2" part is covered by tape on GR's picture) which is German for "container 1 of 2"

I had never noticed that before

So the mystery now is: What was on the second container? It must be the metal stand to view the models for training! The longest component of the stand is a 1770mm long 12mm rod that goes inside a 26.9mm tube that is 1440mm long, and the tripod that holds this assembly has footprint of about 700mm diamater. This is not a small stand

a model on it can be raised up to about 3000mm (10 feet)
One of my sets has a 463 lot # (GR's is #38) so there was quite a number built.