Link to the APG review of the Mercenary
Body kits were the norm back in the mid 80s. Basically, you had the 007 and PGP on the market. So a huge aftermarket developed. Instead of whole new guns, you usualy saw "kits" that you utilized parts from your 007 or PGP into the new body.
Maybe those small parts, like sears, hammers, or valves were too hard to fabricate? But by the late 90s, most "upgrade" companies were selling complete guns. ie Bushmaster, Phantom, SNiper.
Then by the time the Merc kit came out, cheap pumps were flooding in. Ie Bloodsucker, Razorback, etc. There was no longer any market for the kit, which was a shame, since its a beautifull looking marker.