The CVA Mountain rifle has a fly in the lock. The lock internals require a deeper mortise to accommodate the lock parts. The lock plates are the same dimensions, but: The mountain pistol, IIRC, had the cheaper no fly lock and a shallower lock mortise due to the use of just a tumbler. So my view is that you will need to do some digging to gouge out the mortise to use that Siler lock. While you are at it, you ought to install a single set trigger and make a decent target pistol out of it.
BTW, I do not believe the mountain pistol was ever made in USA, only Spain. The first few years, the mountain rifles were made here.
Also, I was not aware that siler made such a lock. I believe L&R did however. They made replacements for the TC guns and at least for a while for the CVA locks with a fly.