I find the "C" rather far fetched as a prop marker or even an intentional c. How are props marked anyway? If it was me the why not R for rock. Then you have medium, large etc so that becomes RM for rock medium. You will have more than 1 so you need a numbering scheme to catalogue all the rocks so somewhere you could have RM 0000000000000000000000001. Then RS (for rock small) 00000000000000000000000000000001. Then there are the colours. Grey, slightly grey, not so grey etc etc. Well, maybe. Then why mark the set with the risk of being discovered?

Thinking out loud. And by no means qualifying myself here.
To claim that it is a prop marker then you need all the back up that goes with a film set for your theory. That includes people. Now IF I was to do it today I do not expect I would do it any different now as I would then. That is to film it end to end for each mission. Then you would have people check and re check everything to make sure the place is clean of anything to give the game away. After all, you are going to try to pull the wool over the eyes of the USSR who will be scouring any images and film for their own satisfaction. You need to convince the rest of the world as well but the race is to beat the USSR.

Back to the people mentioned second paragraph, first line. They talk. I doubt you would find enough in the forces that have spent years practicing slick filming and production techniques. This is not a swipe at the forces, just that they do not do this as an every day job. So the staff on a project like this have family. They talk to their families and I think you would be able to find out when staff were away that coincided with a launch and trip to the moon. "where you off to dad?" "cannot say son, back in three weeks". Which raises another issue if you think about it.

Then the USSR. They did things like track the launches as well as many amateurs. Many staff at the remote tracking sites were locals. So is you think about it, you can launch a rocket with three men in it. Take it a bit further you can get it around the moon. A bit further again you can orbit the moon and drop down to land. Why cheat on a sound stage? Also very hard to cheat with the rocks as well. Impossible?