Conn serial # question

[Bach5G]

I was looking at a Conn with a serial number of GG 770###.

According to the sources I looked at there were GA prefixes in the lates 70s but I didn’t see any references to GG. Can anybody explain?

The vendor thinks late 50s but I’m pretty sure that’s wrong.

[Vegasbound]

Certainly not 50’s prefix letters started early 60’s and the GA early 70’s

70 serials 1977

Maybe post some photos and that will help identifying it

[Posaunus]

According to the Conn Serial Number listing, the 1977 instruments were numbered GA700000 and up.
1978 started with GA800000.

Could the "GG" be a typo or a misreading of what's stamped?

[Bach5G]

I see Dillon identified a couple horns as GC and, I think, GJ.