XTX200 and XTX50 are the
exact same second stages except for some cosmetic differences. The XTX50 also has a micro adjuster, as
@Pao has explained.
If you want to get really technical the only difference between them is:
200 has a "chrome" purge button -- 50 has a black one
200 has a "chrome" front cover -- 50 has a black one
200 has a black inner cover --50 has a chrome one
200 has a XTX200 decal -- 50 has a XTX50 decal
Other than that every single part from the XTX200 and XTX50 are the exact same with the exact same Apeks part numbers.