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Why black (there is/was no color transfer as the linen pages were white
If this is a colour or more likely tarry substance transfer (looks like what I had seen before, partially white paper on back) then it would not have come from your aunt's album pages, but from the parcel wrapped shut by oily tape of some sort. Higher denomination stamps would have been used on parcels.
Because there is no thread visible then it may be a forgery. What is the consistency of the paper? may need to be examined by someone used to handling these stamps (not me, although I would love to examine it as I am interested in these numeral stamps and their cancels.).
There are books on forgeries by different forgers, possibly at your library, some I think online her and there. There were and still are black and white stamps. Cheaper to produce, Coloured ink would cost more.
It may just be a sample produced by the Post Office for training purposes as suggested above.