In retrospect I think that you are correct in saying that it fits some Milbro rifles of the era. I looked at the Cougar Rearsight as being one assembly, when in fact it's a two part assembly consisting of a mounting block and a pinned on rear sight. On separating the two it does indeed look as if the sight part of the dual assembly would fit some milbro rifles.

Thanks for pointing that out to me,

Vic Thompson.