Looks British SA 80 ish to me.
No, the problem is that the AR10 is in .308 for STANDARD nato cartridges. This one is for rounds tat are the same LENGTH as a .223, but are the DIAMETER of a .308 cartridge. It's like a .308 kurz (short).
That was my hang up. I originally thought waffle Armalite mag. But it's a short action wildcat mag.
The 30 Remington would be too long I'd imagine so there goes one idea...
30 Benchrest (30 BR) is a shortened 308.
308x1.5-inch Barnes is another.
30 Remington AR (30 RAR) is a shortened necked-up 284 Winchester (But that's only been around since 2008)
The 308x1.5 Barnes and the Benchrest family (usually 6mm BR) have been around since the 60s. My guess would be that the magazine was intended for those.
Does the magazine fit in an AR?
Another pic, this one looks closer to what you have, as far as the pattern goes.