Recently purchased a neglected 200X (1984) to restore. Excited when I saw all these listings on Ebay, Amazon and actual motorsport retailers for rear calipers that state "exact fit for ATC200X, 350X, etc."
Got one only to find out that there ARE differences and it IS NOT and exact match...which caused much frustration. The major difference is the location of the bleeder valve to the banjo fitting. On an OEM caliper, the bleeder nut is located to the aft of the banjo fitting connection making connecting brake line easy and access to bleed also easy. However, these knock offs have the bleed valve adjacent to the banjo fitting connection. Once hooked up, the banjo fitting is SNUG against the bleeder valve making it almost impossible to open / close. And if you do, most likely you'll round off the nut.
Then, there's these "universal" fit rear master cylinders. With some jury rigging you may get them to work but they're not great. Low quality. Whatta ya expect for like $20.
Anyway, just wanted to say "buyer beware and be prepared" if you have to go this route to get your ATC running. Sometimes it's best to pay the crazy cost on Ebay for an OEM and repair / rebuild it than getting the cheap stuff.
Good luck