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For sure they were - our neighbour had one in perhaps late 1970's. I have and have had multiple in 243 Win and 308 Win - as you say, single stack 3 round detachable magazines - but latch mechanism like a "button" on lower rear of the magazine - so it went with the "clip" - the release "button" was not part of the rifle.
Two pictures of a Rem 788 magazine - might be able to see on first one that it is marked on the side for 243 Win. Note the "button" for release. Bottom appears to be simply folded over metal - is no "rim" or "rail" like in OP's pictures.