Tom and I were thinking that these may be the two letter prefix on your gun's serial number. Only problem is AA is 1975, and the gun wasn't introduced until 1979. I also don't think I've seen a gun that wasn't "AA."
Any gun made in Europe must be sent to a proofing house where it is fired with a heavy "proofing round." If it doesn't blow up, the house will stamp it's proofing mark on the gun. On an HK pistol, the mark is applied to the barrel and the receiver.
It could be this two letter code is hiding on the barrel, under all the sheet metal.
Dean tells us: "After checking out the date codes, it looks to me like
the code is on the bottom of the receiver just forward of the loading ramp.
I have an older style SPAS and the first symbol is an AI in a box. That would be 1986, which sounds about right."
That sounds more like a proofing mark!
Thinking about it, the date code would need to be applied by the Proof
House, so it couldn't be part of the serial number.