I have a Fist holster for my XD9 service and while I don't have a sub I I'm pretty sure the sub would fit any holster made for a service model. I think the guns are pretty much identical except for the grip size (which wouldn't matter anyway) and a shorter slide and barrel.
FIST holster are custom made to the handgun you order it for. So i would guess that one holster would not necessarily be good for a service and a SC. The barrel and slide would not come down deep enough for a secure fit. I think FIST uses those plastic models like the ones at LEO academies to mold the leather around. Just my thoughts.
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