It isn't just centerpulls that used those, they're also found on sidepulls. Any brake really, that mounts to a round seatstay bridge with no flat spot built in. Some better frames had the flat spot that replaces the radius washer going way back, '50s or earlier? But they didn't get popular until maybe mid-'70s, and became ubiquitous on any but the cheapest frames by the '80s.

Here's my 1960 Specialissima with the flat spot brazed on:



Because they gradually became a thing only for cheap bikes, if you want a high-quality one, look for older ones. Quality mostly just means nicer finish; I don't think there's anything wrong with the cheap ones structurally. They just look ugly. But who looks at your brake radius washers? OK I do, but I'm weird.

The ones on Weinmann brakes (sidepull and centerpull) in the '70s were nice, and came on Schwinn bikes, and Schwinn dealers were usually good about stocking spare parts, so Weinmann radius washers are generally easier to find than Mafac. But they're all interchangeable. There's nothing functionally different between a Mafac and any other brand, that I can think of anyway.

EDIT: Oops, OP is from England, so Schwinn dealers are no help, sorry about that! But Weinmann brakes were common everywhere, right?