Top: SKSNR... very late production SKS
Bottom: 416 Public Security
Now on pressing fitted parts... no index/witness mark would be needed, generally when pressing, you do not/can not/will not turn one component. pressing parts can require TONS of force, thats... TONS per square inch. I have personally used a 250ton(a press that can develop...a half a million PSI) press several times to press track pins from an excavator, and routinely use a 100 ton press to replace spring pin bushings.. One man can not over come tons of pressure with a simple handtool and leave no mark, much less many tons, you will not move it, the only movement with pressure will be linear with the motion of the press. It's a straight in-line force. One would have to line everything up first and then press. Instead of an index mark, the rear sight could have been installed on the barrel first and then the barrel set in a press jig to hold both in the correct orientation, or just to get the process going and then pressed together.
If by chance you started the process of pressing, made it in say 1/8 inch, stopped, decide oh, I needed to turn it a hair, and you did manage to move twist one of the components, you could gall up the mating surfaces or simply put tear the metal from both surfaces and make both parts unusable. Which ever is the softer material will most likely be damaged, if both parts are of equal hardness, both parts could suffer damage during a twisting operation. If one is lucky enough, you may get lucky and your safest bet if something is off is to stop, reverse the pressing process, press it back apart and start over.