Most backfeeding problems are caused by worn cylinder bars or the front concave being set closer than the rear. I'd suggest you check these two items. Hope this helps.
I have a 1972 510 book and the front beater was an option. So it could maybe be removed. Some things have stripper plates on top to keep things from rapping and backfeeding. This could maybe be the problum. See if it looks like one may have been there. Also might find good beater blades cheap at a combine junk yard.
I will check those 2 things if I have time some day. I am going to try it in wheat to see if it backfeeds. If it doesn't then I will leave it alone. I am collecting combines and this combine won't be used very much. I want to cut enough this year to shine it up.
I have been using 410's 510's and 550's for over 40 years doing custom work for a lot of those. My advice is to throw that front beater away. Just take it out and the feeder house will go right back on without it. Will work great without it. Also make sure the front iron on the concave is straight and in good shape. Goog luck.