Legacy (aka Mod 0) guns had a CHF CL barrel, gas block installed via set screw, URX2 10.75" handguard (PNs: 20549 and 23375), and the QD cup lower. Some Mod1 guns also had the QD cup lower. The trigger in the legacy rifles should be adjustable as well.
The silver guns you see were also produced in green, tan, FDE, and black.
The gas system length is almost the same as the intermediate but it is actually .4" shorter than the VLTOR/Noveske Intermediate spec. You will also occasionally find SR-15 E3 stripped uppers for sale which do not say "Mod 1" on the side, and you may or may not get one which has the KAC crest near the CH.
The URX2 10.75 includes a flip front sight directly behind the gas block, and you can see both PNs here: