Nothing has happened to these stamps in the past 40+ years except for some minor "conservation" by removing them from proximity to foxed paper. The page fragment with the 1/2d Keyplate on it was torn through the corner of the stamp, which is why it is missing a corner. The torn portion was not present in the group as purchased.
While they do look amateurish it would seem unusual to me for specimens to be faked 45+ years ago. I well remember that specimens and reprints were considered little better than "album weeds" back then, and the prices were commensurate (see scan below).

For those of you who don't recognize this distinctive handwriting, this is a TPS sale-by-tender backing sheet from Jim Petterd [noted Hobart dealer], for a lot he sold in the late '60s-early 70's, which I won. I hasten to add it DOES NOT refer to the specimens in this thread. It did however include a 3d St George '79 reprint with the overprint inverted, plus the 1d and 3d platypus with horizontal "Specimens" - nice items!
I bought most of my specimens and reprints from him, and his going price was 6d-1/6 for any Tasmanian specimens and reprints. I started collecting them because it was a cheap way to own examples of stamps I could not otherwise afford as a kid. What I'm getting at is, why weren't postmarks faked onto these stamps rather than specimen overprints - surely a nice used 10/ Keyplate or St George with a postal cancellation would be worth far more, even back then? I suppose the answer will never be known.