The TOP mechbox is generally similar to a normal gearbox except for the compression mechanism: the piston, cylinder and cylinder head are replaced with a bellows:
You can see it compared to other setups:
Unfortunately the pistol and bellows were known to fail fairly fast on just about every gun. ANGS made some upgrade parts (a large cylinder, also pictured above, and associated piston, etc). PGC made a complete drop-in mechbox replacement for the M249 (which was copied into the CA and later A&K 249s) but I don't recall them making one for the M60 (a quick search will confirm either way). The PGC mechbox used standard V2/V3 parts, but the ANGS kit had a lot of proprietary stuff iirc. (notice the short piston in the pic above, along with the unusual piston head).