I agree with Boris. No doubt in my mind that it's an IC and in great shape at that. Spike cutout in the stock is a bit crude, typically they are a little bit nicer than that, but I have seen the "beaver teeth / dull butter knife" effect on these before. I suspect they were individually hand done and the appearance of the final product a direct result of the skill of the individual doing the work.
I think your suspicions aroused by the gas tube markings are correct. That is a Russian SKS tube. Chinese year 12 /26\ gas tubes had moved on to the two piece design like this from S/N 12203504:
Here's a closeup comparison of the area where the two piece design is apparent that I use in the survey:
I wonder if it was easier to just swap the tube than to try and replace with the laminated handguard or if the original tube had enough issues that it needed replacement anyhow. Either way, 90 out of 100 people aren't going to even notice anything off about it.
Last thing is the S/N at 12,25x,xxx puts it right at the tail end of where they used the normal takedown lever. Inverted takedown lever appears ~12,28x,xxx, so not too far away from the transition.