I think it was proved 2 times before sleeving in 87'

Yes, the initial proof was 1896-1904 confirmed by the 1 oz. MAXIMUM
Bore then was 17/1 = .655" - .661"

54-89 it was proved again, with 2 1/2" chamber for 3 Tons

It was proved after sleeving now with 2 3/4" chambers for 3 1/4 Tons for a max. service pressure of 9,800 psi. That was probably when the bore of .661" was marked.

It is relatively uncommon for the Birmingham Proof House to deface the previous marks from the London Proof House

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