Arnold,
Some of the early Astra 300s in 9mm Kurz were not waffenamt (WaA251) acceptance stamped (I have one). However, the serial number you indicate is in the German contract range (July 21, 1944 delivery) and should have the WaA251 acceptance stamp. The one you are looking at is near the end of production and (just pure speculation here) the Germans may have missed stamping this one in the haste of retreat or the commotion near the end of the war. Check and be sure of the serial number and that it is indeed a 9mm Kurz and not a .32 caliber model which were not waffenamt acceptance stamped. Maybe some other will have some further input and ideas.