What a Difference Bullet Weight Makes!

[Oscar77]

The barrel is marked "C MP Chrome Bore". According to what I've read that would make it a 1/12. There are no other markings anywhere.

Oh........Sorry.
I misread your "......The barrel isn't marked with a twist rate ......." to mean there weren't any markings at all.
Yes, if it has that ( "C MP Chrome Bore" ) then, yes...........I'd go with 1/12".
(The VERY early ones had simply "12" marked on them. The EVEN EARLIER ones and extremely rare ones were simply marked "14." For their respective twists).

Just yesterday, I worked my A1 upper out to a 200yd zero and played around.
Man, those things are light!