OoooOoOh.
The most info I can find is that
Receivers with tabs that open like a real AR:
TM and its Clones
KWA
ICS
Receivers that slide open:
GP
GG
CA
KA
Madbull JP Riffle body
VFC
PRIME
Best Gun MRP
AGM
DBOYS
That should cut it down a little.
But all the sliding receivers likely are different still.
AABB uppers will fit G&P lowers vice versa. AGM uppers will not fit AABB or G&P lowers and vice versa...King Arms uppers will fit AGM lowers and vice versa...
AABB upper receiver with G&P lower receivers fits without no modifications...
also the "BG VLTOR STYLE AEG UPPER RECEIVER " says it fits G&P