Some chains have additional guard links or bumper links and the FOP may not fit those.
Also, some cutters have different geometry and again the FOP may not fit even though the pitch is correct.
There were different FOP's for different chain types (Chisel vs. Semi-chisel) and one may not actually fit a different chain type.
These all fit some form of full size 3/8 chain. I had the blue ones made because 37509 was put of production for 20+ years and no longer available elsewhere.