That is definitely a Redfield base. It is a later one with the contoured corners. It takes standard "turn-in" rings that are currently made by several firms, Leupold, Burris, Weaver, to name a few. They are availlable in several heights, horizontal split, blued or matt. They are very common: You can get them from ebay, Amazon, Gunbroker, etc. or your local gunshop. The vintage "thick/thin" rings would be appropriate but certainly not necessary for that rifle. An older Weaver K4 would work well but an old Weaver K6 would be perfect. The .250/3000 is one of my favorite cartridges and is mild enough to be a good fit for that low numbered receiver.