IronLance,
On a FAL, (or L1A1, C1, T48, R1, G-1, StG-58, ect...) the furniture attaches to the barrel and the lower receiver.
While Imbel made metric receivers you can screw almost any barrel inch or metric on to one and attach almost any lower receiver, inch or metric to it. The only problem comes with regulating the sights.
The barrel on nnf's rifle appears to be inch with open sight ears. The lower also appears to be inch with the folding rear sight. I believe Century had these Imbel receivers cut to accept inch mags, so it is kind of a gray area as to "Type" on this receiver. "FrankenFAL" describes them best.
That coupled with the fact that Century was using Brit kits to put together the "Butthole" stock and no muzzle device L1A1's to make rebuilding them legal (at the time). The kits were supposed to be in pretty good shape on these rifles
The handguard can be replaced with an inch or metric version, but the buttstock and pistolgrip can only be replaced with inch furniture on inch lowers. The way the pistol grip attaches also MAY have been altered from standard inch to accept the BH stock on nnf's rifle.