Out of curiousity, how far off is the HK51 carrier from an HK33 carrier?
I see these at RTGParts...
https://www.robertrtg.com/store/pc/UPPER-TUBE-FOR-HK33-BOLT-CARRIER-UNFINISHED-111p5978.htm
Could one, in theory, cut the tube portion of the CETME carrier off and replace it with one of these...