Question for someone who knows about brass?
Answers:
The reason a magnet sticks to the back is that there will be two steel bolts (and probably a steel spring for the flap). If the letter plate really is brass (yellow metal, soft and bright when scratched) then it may have been chrome plated.
Rechroming would be expensive because the metal would have to be repolished before it could be plated (the finish of plating is only as good as the finish on the metal underneath.)
One can often find nice old letter plates in antique/junk shops or at Sunday Markets/Car Boot Sales. You might, at least, keep an eye open for something of the same era as your house (or maybe a little older).
Edit: if the silver coloured metal really is under the brass, I am puzzled. There were letter plates of that era made out of what I think is called gun-metal - a greyish metal, fairly hard but lighter than brass or steel. They were usually chrome plated and the chrome very often deteriorated whereupon the plate was usually painted.
Brass is an alloy of copper and tin so it's not usual (and I don't think possible) to 'brass plate' - at least, not by electrolysis.
Other answers:
Any hardware store has brass cleaner. Most of it works well.
Any hardware store has brass cleaner. Most of it works well.
DO'NOT use silver polish, how ever try using "citiri-surf 77" comes in a pump sprayer, you can get it from a company called STELLAR SOLUTIONS, it works a treat