I need help. I am unsure about this cancel. For me it looks like a fake.

You are probably correct. The high value stamps were normally used for accounting or telegraphic use. In the past purists often insisted on postally used copies and the original single circle cancellation (which is almost certainly genuine in this case) was probably embellished to give the appearance of a postal camcellation.

Ray

The cancel is OK. Belfast was one of the more common cancels on the £5 orange. You should assume that it is not postal use. The bars, in addition to the cds, is fairly common for Belfast.

None of the £5 Oranges with a single central cds cancel is postal use, however attractive they may appear. Postal use was rare and always has more than one cancel. Equally two or more cancels is not proof of postal use.

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John Horsey
County Philatelic Auctions
https://www.stampauctions.co.uk/