The muffler you have is the "twist'' muffler, named after the twisting chamber the unit uses for quieting.

It is not adjustable.

It is restrictive at high revs, but shows less loss with props toward the larger size.

If you use it, be sure to support the rear of the muffler. That is what the set screw and hole are for. The housing is crimped onto the inlet and vibration will cause it loosen quickly if the rear is not supported.

A better choice for performance and no hassle is the UltraThrust muffler from Performance Specialities. Reasonable quieting, better power, and no support wires to deal with.