260rem,
I bought the rifle used; it was the owners deer gun in Iowa. The hardware on the scope is a pistol mount and the hardware fastened to the rib with 3 screws is custom. It is not easily removed as the former owner hunted deer with it for 10 years before his untimely death; a heart attack while dragging out a huge buck.

Daryl asked "Nice - what, pray tell, is a North Fork 350gr. SS - is it a .458 dia. bullet in a sabot, or is your rifle a .458 of some sort?" It is a .458 North Fork semi spitzer bullet.
https://www.northforkbullets.com/458-350.htm

My Beretta rifle is a .45-70 (.458 2.1) and each barrel is MOA accurate. 100 rounds of 300 grain ammo came with the rifle, but it did not regulate well. So, I began playing around with heavier bullets and found that they shoot/regulate much better. The NF 350 and Remington 405 grain bullets show promise as big game bullets.

I also intend to shoot some of the 350 SS bullets in my .45-90, but at higher velocities than in the DR.