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The 50, 100 and 200 are not the same. There are similarities in the 2nd stages, but the first stages have some minor differences:
50 - The 50 can have a DST 1st stage (swivel) or a DS4 1st stage (no swivel). The DST has 4LP & 2HP ports. The DS4 has 4 LP & 1 HP port. Some people don't care for the DS4 doubles routing with certain tanks and manifolds.
100 - In the 1st stage the 100 (FST) is a little larger and heavier than the 200 (FSR) but has exactly the same hose routing. Both the 100 and 200 have 4 LP ports & 2HP ports.
200 - The 200 has good hose routing, but isn't as low profile as the 100. It has a removable valve seat that the FST doesn't have.
100 - In the 1st stage the 100 (FST) is a little larger and heavier than the 200 (FSR)
The XTX50 with a DS4 is a phenominal workhorsel