"If we had to pay to walk... we'd all be crazy about it."
--Edward Payson Weston
My main protein source: Lots of peanuts, good for efficient calories, protein and salt. I keep them in a separate bag from trail mix. Also, I grind them at home in the blender to make a powdery paste and then reconstitute them on the Trail and serve with rice. Of course if you're thruhiking, that only works if you're doing maildrops.
Also I used to pack powdered milk a lot but I've phased away from it.
Summer sausage, like regular SPAM and Vienna Sausage, has a great deal of fat, especially saturated fat, that is in place of the desired protein, that a hiker pays for, and lugs, without payoff. All of those products would IMO be more accurately labeled as "Lard garnished with (low-quality) meat". I suggest hikers instead buy meat with several times as many calories from protein as fat. That would mean for land animal meat, pick leaner beef cuts over fattier ones, bison or venison over beef, beef over pork, poultry over mammal meat, etc. I have also added small Ziploc bags of egg white powder to nearly all my mail drops for my upcoming thruhike to add to casseroles to raise my protein intake, and suggest others do this.
Fish and shellfish of all types comes to mind for the best protein/sat fat ratios, though. On fish, try to pick ones with Omega-3 oils (but from smaller fish than tuna/swordfish to limit heavy metal contamination) preferentially. On shellfish commonly available in hiker-convenient forms, clams are arguably nutritionally tops IMO (less cholesterol than in shrimp if memory serves, and essentially no fat, which you want to be getting from nuts and olive oil).
P.S. on beans as a protein source during hikes...
1) Dehydrated ones make way more sense to carry than canned ones. You can pick up water along the way; why pay for it and carry it, in heavy cans?
2) Beans are not complete proteins. They need to be eaten with a source of essential amino acids they're low in (such as lysine). Rice is a good choice, but some animal protein (meat/milk/egg whites) makes a bean meal even better nutritionally.