i'm sure someone will chip in with a likely value - but without pics hard to identify which one, and hence relevance to attaching to a 'vintage' airgun.
If you have a look at the link below, it shows the range from an original brochure. Bearing in mind the different sizes (which are obviously intended for different rifles) - which is it?
http://www.rifleman.org.uk/BSA_sight...sories.htm#No8