It appears to be the only connection between the bus bar and the service feed.
There's the trouble.
Right by your mark "C", there's a screw, and an obvious horizontal bar about 1" (25.4mm) wide. You can see it on the left side, also. That bar actually goes all the way across, and forms an inverted "T" with another bar coming down from the neutral lug at "B".
The small screw at "B" is not a terminal intended to mount wires. It appears to be fastening the neutral lug to the neutral bar, the "T".
If for some bizarre reason the netural bars as provisioned could not provide the needed service, and someone needed to add another wire to the neutral lug, then they need to use a different lug which is listed for the purpose. Double-tapping a lug is a code violation unless the instructions say you should do that, and you conform with the metal type and size restrictions in those instructions. UL approves instructions when they approve equipment. Assume the factory neutral lug was approved for one (1) wire.
NEC requires 1 neutral wire per screw on the neutral bar. Grounds can be combined 2-3 if the labeling says so, and you can always add accessory ground bars that attach to the enclosure steel. There are no accessory neutral bars. You have enough neutral spaces, so says UL.
No screw should be messed with without the use of a torque screwdriver to set torques to proper spec. This is true of every screw in this enclosure, except the screws which mount the bus assembly to the outer box.
However, the half-wits who put these weird straps in there were obviously having some sort of a problem. No idea what the problem was. But don't be surprised if removing them re-introduces a problem they fixed unsafely. I hope you will chase that problem, then, and arrive at a proper and Code legal solution.
If you reach a point of despair with the panel and must go to a new panel or simply want more breaker spaces, take careful note of the make of all costly (AFCI or GFCI) breakers and get the panel make approved for the greatest number of them. E.G. The correct breakers for an ITE panel are Siemens, and assuming most breakers in it are ITE/Murray/Siemens, get a Siemens panel.