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Am I correct in assuming that this item has been cut to shape from a GB postal stationary envelope?
Yes (stamped to order). It was possible to have the 'stamps' impressed on private stationery, well into the 20th century.
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If so, was the surrounding collar printed at the same time as the indicia (by the GPO) or was it added privately later?
The printing of the 'stamp' and collar was done at Somerset House.
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Judging by the matching colours, I would guess the former.
Both can be found. In this case, it looks like the collar was printed at the same time as the 'stamp.'