Hi,
I'd put money on that frame having been built by Les Ephgrave - a frame building legend.
Les was well-known, for using brake bridges of exactly the same (diamond shape) on many of his bikes.
The image is of a brake bridge on an Ephgrave frame.
Keep the bike, don't re-finish the frame - just give it a wipe over.
You've got a very desirable bike frame there - do some research on Les Ephgrave - he was a frame builder of the highest renown.