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 !!
Do you have any history about the lamp ?? Where your parents got it and what was told when they bought it ?? I am not sure and no way to Identify it without a makers name but I do not think it was American made !! My feeling is that it was possible made in France !! I have a friend on another forum that collects French antiques !! I am going to get her opinion on it !!
Are you in the U.S. or UK ?? Are some of the circular prisms at the bottom missing ?? And one more pic please,, can you show the plug on the electrical cord ??