I have the Sarco rear sights ,to go on my 375 2 1/2 " project.
They look a bit rough in the pic on the website but are nicely finished in the flesh .Looking at the 1912 BSA catalogue the 375 had one standing 100 yd leaf and two folding one ar 200 and one at 300 yds.
The sarco sights come with an island base but I reckon it would be easy to remove the sights and just dovetail the barrel and fit the sights directly into the dove tail on the barrel.Depending on the finished hieght they would look more original.

Kev.