Lighting in general is almost impossible to identify unless it has a makers name !! Even then it is difficult to say for sure !! Problem is that only in very rare instances are all the pieces made by one mfr !! Normally the pieces are ordered according to the company`s design and are simply assembled by the company and tagged with their company name !! This holds true even today !! I saw nothing in your pics that looked as if this fixture is a conversion so I think it was always electric !! From the size you stated it was likely an entry way/foyer fixture originally !! Hopefully we can at least get an idea of where it is from !!
Another bit of information for you since you said your mother bought it at least 50 years ago !! Back during the 1970`s and 80`s anything labeled Antique was bringing top dollar here in the U.S.!! Buyers were going to the UK and purchasing almost anything that looked old and shipping them back here for sale !! I had my shop then and frequently bought some of them !! They were paying pennies on the dollar for furniture lighting and other things !! Quite a lucrative tactic back then !! Thats why I think your fixture may be of French origin since it is just across the channel from the UK !! It is not unusual to find items from France in England !! Let me cross post this and get other opinions on it !!