Challenger Plainsman pneumatic pistol.
The information below is taken from the catalogue advertising the auction of the Myron Kasok airgun collection, sold by the Rock Island Auction Company December 13-15, 2003. Pic is copyright Rock Island Auction Co.
Thanks to Rob for these pics:
Challenger Plainsman pneumatic pistol
With thanks to D T Fletcher for the heads up on this pistol advertised on Ebay.
Boxed Challenger Plainsman pneumatic pistol