The fastener looks like a very crude tri-wing security screw profile Nick. Could very well be proprietary because I've never seen a tri-wing with a raised center like that. Is that a solid state radio, or does it still have some tubes?
It's a solid-state Hong Kong model typical of the late '60s into the '70s.
Finally was able to disassemble it yesterday. Those anti-tamper screws lived up to their name as it was a very long, patience-trying process with a small side cutter and needle nose to get them out. They laughed at my attempt to drill out the center and because they are recessed there was no access to the edges.
The outer plastic case has yellowed and is brittle from age and exposure but I'll give it a good cleaning and polish. I don't expect much from the radio as it appears to be a single transistor so performance will be marginal but the electrolytic caps will be replaced along with the burned out fluorescent back light bulb.
I found out Peter Max is still alive ->
https://petermax.com/More to come!