Now that looks familiar. Got one just like that. Williams sight are what came on those so you'll be able to find what you want.

What caliber is your gun?

Who made it?

The one I have is stamped White Armstech cal .451 20" bull bbl serial #318. Unfortunatly for me the previous owners didn't do a very good job of maintining the gun and the rifling is just about non existent.