I wo0uldn't call them bogus or fakes, because I do not think there was any intention to deceive. I suspect they were produced as a souvenir for a meeting/convention of the Society. You see a fair number of Canadian stamps that have been overprinted for that purpose.
I believe that these overprints are perfectly legal, and were applied as a convention "event" marking. The US version of the PSS does the same thing every year at their national conventions (or almost every year). They are well within the "legal" markings that can be applied to stamps (as are perfins). One can do pretty much anything providing they fall within a set of the country you are ins' parameters.
These stamps are among the most common in existence and they haven't been valid for postage since at least 1924. Anyone can take them and overprint anything they want on them, legally. It might become illegal if someone then tried to sell the overprint as some kind of rare item worth a lot of money, but the simple act of overprinting them as a souvenir of some sort is just fine.
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