Agree, does not look like the typical RC "X" stamp but I have no idea what the stamp means or who used it.
I think the sling is a post war G1 sling with a reproduction sling buckle. If the sling only has two holes for the buckle, instead of three, it’s a G1.Welcome aboard.
We see a screaming RC, but their shellac has been mostly stripped-off the stock (from the pics, traces of it remain).
The Russkies tossed the front sight hoods, so that bit was added - & it's likely a repop.
The sling appears to be WW2 German pattern, at least - but is way too long for a K98k.
I think the sling is a post war G1 sling with a reproduction sling buckle. If the sling only has two holes for the buckle, instead of three, it’s a G1.