Interesting, I never saw a Mark X in .223, just the F.I. "Mini-Mauser" (which really isn't.)
You got pictures?
Edit to add: I found references to the "Mini-Mauser Mk X" and suspect that is what you have, even though it is different from the regular Mk X in almost all respects.
The whole family was later sold by Charles Daly and now Remington. Unfortunately, the Remington catalog and owner's manual "specifications" do not incude the twist rate.
You can determine the twist rate by running a tight patch down the barrel on a swivel handle cleaning rod. Make a mark on the rod and measure how far you have to push the rod for it to make a turn.