JMOORE is right on the issue of the Tokarev muzzle device.

That gizmo LOOKS like the type fitted to the Russianicon SVT-38 and early versions of the SVT-40. (Later SVT 40s had a simplified brake with two large ports either side.)

However:

It would have been seriously "abbreviated" to fit on a No4. The original SVT brake is part of a long sleeve that extends back to and incorporates the gas regulator / base for the captive piston. It is threaded onto the barrel and locked with a key that looks a bit like a "rounded" version of the barrel wedges on Colt percussion revolvers.. The catch is that this retaining thread is way back near the gas-port end.

Furthermore, the front sight post looks odd. The original sight on the SVT series was a threaded post, reminiscent of those fitted to SKS and AK types.