Thread: Can anyone identify what action this has been built on?

That style of barrel nut is a bit of a fingerprint for Savage (or Stevens if you don't want to blame Savage).

There were a couple of other makers that used a barrel locking nut to lock the barrel at the correct headspace measurement, but the only one I know of using that style nut is the Savage line. Probably a very good shooter of all things are right. The later models of this rifle used a more traditional looking affair that used either a splined spanner or you could sometimes get away with a c-spanner to undo them.