Mis-matched mutt is a good way to put it. The carbine stock and handguards (and possibly the barrel) have been added, replacing the original parts.
I note the forward assist has the early "teardrop" shape plunger, which, in my experience, snags on everything (and sometimes seems to on nothing
).
In the interest of precision, I need to point out that the bolt for the select fire and semi auto guns is the same. The bolt carrier is what is different.
The weathering and wear on the pistol grip say its original, to me.
Looks like a nice example of a modified and slightly worn rifle, to me.
Congrats!