Can you post a picture? I have never seen a "Gib's Summit Carbine" let alone one in .45/70.

It is based on a Siamese Mauser?

You could Google Express Sights. There are a couple suppliers State's side. Brownell's has the one I used on my 14 bore rifle. It was $96.95 when I bought it in the white, in 1986 - but not much more than that now.