Very interesting article from the Historical section, at least it establishes the date of the Ealing factory when it was built. Below is a link to Patentscope as the IPO claim all patents from 1900 are now on WIPO’s Patentescope, however past experience with the IPO does tend to make me believe that not all may have made it to WIPO’s system as the IPO has quite a backlog and shortly I suspect things might get worse there as there are rumored up to 40% job cuts.
https://patentscope.wipo.int/search/en/detail.jsf?docId=GB137211095&_cid=P20-L586LC-70772-2
As you will notice the application number and publication number is longer than G328Y and prefixed with GB. If G328Y refers to the patent classification the problem is even more weird. There was (now superseded) a G classification for instrumentation, optics are one of the subsections under G, but there isn’t/wasn’t a subsection which bears any resemblance to G328Y
The Patentscope entries for Ottway only fill up 3 pages
https://patentscope.wipo.int/search/en/result.jsf?_vid=P20-L587CK-97814
Of course the G328Y if it is a patent number could predate the Ealing factory, since they put this on their gunsights it would be interesting if the origin of G328Y comes to light.
The link below is a guide to finding GB patents from 1617 (GB1) to 1899, after 1899 in theory the patent should be found via Patentscope
https://www.bl.uk/help/find-early-british-patents
Good luck
Daryl