M77 Magazine Box Differences Question

[Black Fly]

My old M77 ST in 257R has never fed the 3rd and 4th cartridges without pushing them around with my fingers. Its been that way since I bought it in '72. This morning I was switching out the stock on my RS 06 from the 80's and I noticed that the magazine box was very different in the two rifles. The ST box is basically a rectangle, while the 06 box is strongly tapered with a slight flare at the front. Pictures down in my WTB M77 Maggazine Box in the Classifieds. When I put the 06 box in the 257, cartridges fed smoothly. When I put the old box back in, the old problem returned. Im now looking for a box shaped like the one from my 06.
I looked at my short action M77's and they have the square box and feed just fine. So was the magazine box for the long action changed to correct feeding issues? Second question, are magazine boxes from Mark II's compatible with tang safety rifles. Or am I just chasing unicorns?
Thanks
Bfly

 

[Black Fly]

Here are the two boxes. The top one is from th RS from the 80's and the bottom one is from the 70's ST.

 

[Neon Vernon]

I've never tried swapping boxes from the tang rifle to a MkII but they are quite different.

The 3rd or 4th round feeding hangup is likely a spot where the follower is binding on the inside wall of the box. Look for a rub mark and polish the box inside and side of the mag follower accordingly.