With the advent of the Pmags, the Colt 5/20 or even 5/30 mags seem to be more of a "collectible" these days. I remember when the NIW 5/20's were selling for $50 or more a pop - and nowadays, they're going for between $25 to $35 depending on condition (with slightly higher prices going towards the factory "U' blocked and rivetted 5/20's). With regards to the carry handle scope mount... Again, not too many people are rockin' the fixed carry handle these days - let alone putting scopes on them. So your mount might be slightly undervalued given the current "tacticool" market... If it's in decent shape (especially if it's a Colt branded A.R.M.S. product), it is worth what someone who collects Colt stuff will pay for it...
hsld.