Longshot info request on MiG-21 of 921st FR, NVAF

Messerschmitts are my usual bag, but I’m doing research for a long-term modeling project involving the 10 May 72 throwdown between the USAF, USN, and the NVAF. Obviously, info is somewhat scarce on Vietnamese MiG-21 ops during the war, although it seems this situation is getting a little better over the past few years.

My question concerns the aircraft of Nguyen Van Ngai, shot down by Lt. Curt Dose of VF-92 as he scrambled from Kep in the first engagement of the day. I’ve found several photos of 927th FR MiGs, as well as some others which might be 921st or 923rd (can’t tell them apart)… would there be any way to know what the side number of Nguyen’s aircraft would have been? And while I’m at it, do those side numbers have any particular meaning within a given unit, or are they simply inventory numbers within the overall AF structure?

Thanks in advance for any assistance on this!

Lynn