Hi Paul & Alan,
I will be restoring it though it may take me some time to clean and remount it.
There are a few inscriptions, the easiest to read being “TURN TO CHANGE POWER” and “TURN TO FOCUS”. Part of the latter is visible in the image above. Neither of these give much clue to the provenance.
You are quite correct at suggesting Ottway & Co. however, It is fairly easy to read, though the black paint needs renovation, “GUNSIGHTING PATENT V.P. <illegible> W. OTTWAY & CO. LTD EALING 1912”. Another inscription is “PATT. G328Y” and a series of numbers “5 7 9 11 13 15” which are undoubtedly the magnifications available. The only other inscription is “W A/T” where the W is twice the height of the A and the T, which themselves are spaced vertically with a horizontal bar.
It is possible than <illegible> may become readable with some careful photography under oblique lighting and with image processing of the result.
Now to track down the Patent number …