Hi John,
As per PM exchange, AWC was the Serial No prefix used for the 635 Magnum rather than the Challenger. Both were made at around the same time and both are desirable today. The 635 was available in Standard and De-Luxe format, the difference being in the grade of stock. I have a vague recollection that either the 635 was developed for the .25 Milbro Rhino or the Rhino for the .25. Or maybe both were developed in tandem as again, they were both marketed at the same time.
A very nice rifle nonetheless and I wish I'd kept my own boxed example that I bought from Vandeburgers in Tooting. My old no was AWC 02357 which is very similar to yours!
As regards value, I think Troubledshooter sold one on here recently but that was boxed and as new IIRC. Not sure if yours has the original open sights (the rearsight was used as a peep, just like the Buccaneer). The bluing on yours looks excellent and stock is not too bad with just a dew minor marks from what I can see. If it were mine, I'd price it at £180 but that's a bit of a stab in the dark and I'd be interested in what others think.
Kind regards,
John