They used a modified AR-10 mag. Back then the old aluminum waffle AR-10 mags weren't used in anything really. I bought 50 of them from Paragon for my Sudanese AR-10 DIRT CHEAP at Knob Creek back when the HAC-7 rifle was still in production. Holloway took the waffle mag and cut a notch in the back spine for the paddle release. Once Knight came out with the SR-25, the mags became more valuable and the mag ban wasn't much later, so they jumped up to $100 each and you could sell them in minutes online in the early days of the internet.
I would get one of the current production AR-10/SR-25 mags and cut a notch in the spine and see if that works. The notch was simple. It was two sided, i.e. like it was cut with a triangular file. IIRC, the top of the notch was flat, in that it was parallel with the barrel line. The other leg of the notch was down and back. I'll warn you now, I'm going off of memory from 30 or so years ago. So you might want to really figure out the angles before you go hacking away, but it wasn't like something you needed any kind of equipment or real skill. Just a triangular file.