Looking at the action, Someone has opened up the rear of the action to accommodate for a long cartridge, It's possible this action was not originally for Ultra mag.
Like I said in my original post, I opened up the action myself for the 3.825" Wyatt's box. This is indeed a RUM action as evidenced by the cutout in the left rail.
I have had great success doing this on standard magnum M-70 actions and getting them to feed reliably. I had originally thought an M-70 RUM action would be an even better choice to modify, but it's not.
I even discussed this with Shawn Carlock a while back when he came to my shop--as he does from time-to-time. The first thing he mentioned was the issue with the cutout in the left rail when opening up the magazine well.
That cutout in the left rail is fine with a non-modified RUM action. The shoulder of a RUM case is forward or nearly forward of the cutout so the left side case doesn't pop up. However, extend the magazine opening back .230" and the use a 28 Nosler case, and you get the issue I show in the pictures.