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It may be a early version of the TUSA TR-200. There is a schematic on frogkick.
If there is no hole to blow it out, then it is probably a destructive removal. It really should not be removed unless you plan to replace it anyway.
I believe your regulator is a balanced. ....
I believe your regulator is a balanced.
Once the piston is removed you should be able to push on the seat from the piston side with wood or plastic dowel smaller than the piston stem.