I have an HK21 from RDTS and had him weld on a set of Weaver style tabs so I may mount a scope to the gun. Ralphs web site refers to using the A.R.MS. Quick Detach Throw Lever Picattiny Rail mount for this application. Not giving the model # of this unit, where it may be purchased or answering his phone/e-mails, I just don't know where to get one. Looking on the net I came across a few that give little info, all seem to be about 5" overall. My tabs are 5 1/2" on center. Anyone else come across this problem? Can the rear site remain and the carry handle remain while using this mount?