This is looking quite like what I first suggested, which was a fiendish experiment by Ampeg in making things smaller probably because somebody important to them, asked them back then if it was a possibility.
What is the size of a Mach 1 mustang boot exactly.
The speaker surrounds are different to my old flatback.
But the backs of the speakers look the same from memory.
I have some not very standard very old SVT 115 cabs that I love dearly, full of the stepped alnico magneted Altec option, with slighty different porting to other Old Ampeg metal badge cabs I have seen.
Different grill cloth too, with what looks like gold thread running through it.
PS. Nice to have a cool extremely rare probably 1 or 2 off cab like that.
Bet is got some serious clang in the upper mid.