Save your money, just buy a fixed barrel spacer.
There's no reason to use a fixed mount in that silencer unless it's a thread pattern that doesn't fit within the diameter of a piston, and both of those thread sizes you're looking for fit into a piston. Any weight or overall length you save is minimal at best and realistically won't be noticed in use.
Using a piston and a fixed barrel spacer is far superior. Not only will it usually have less first round pop, it'll also make it so when you unscrew the silencer from the barrel you aren't leaving any part of the silencer assembly behind attached to the barrel. Commonly using a fixed mount will leave the fixed mount on the barrel when you uninstall the silencer instead of coming off the barrel as one assembly. That doesn't happen with a piston and fixed barrel spacer because the spokes are locked in place rotationally both directions. That's particularly important to know if it's going under a handguard. It'll also make it so your piston cage won't get fouled up where the piston spokes sit.
