That is the common Weaver side-mount. Note how close the ring spacing is on the Weaver and compare that to the B-Squre.
The split bridge, single locking lug, and genreally crappy triggers have been the reasons that every Savage 340 series rifle that has come my way has subsequently gone down the road to a new home.
Yeah, my old man was a 340 guy. He added a .222 to his 'collection' and installed a Weaver side mount and a K4. The thing was fairly accurate, but not anything you would write home to Mom about. He got it so as to join me on woodchuck safaris. (He lusted after my HiWall .219 Donaldson Wasp, but was too cheap spring for one for himself. We ended up making a deal that he would pop the close ones and I would drill the far off ones.) I couldn't abide the 340's horrid trigger that was only marginally less so after having a trigger job done to it, so it was one of the first to go after it became mine.
I hung on to the 340 .30/30 for a good while for sentimental reasons, but it too went away (to one of Dad's grandsons). Those things are decent entry level rifles but are indeed lacking in a few areas