Hi Moggy,
>I've been researching the british Mk1a Drift Sight, and I think this could be it!
Hm, I believe drift sights are actually navigational instruments, not bomb sights. The bomb sight manual actually demands "avoidance of drift".
Here is a picture of a bomb sight from CW Hewitt's "An Elementary Course of Air Navigation", undated but certainly pre-WW2, showing a simple drift indicator. (There were more complicated devices for the same purpose.)
Regards,
Henning (HoHun)