Richie,
First, Your “C” seems to me to be a curved cut between two tabs, each of which may have a mounting hole for rivet or bolt. There are two others opposite. There also may be a circular hole in the middle of this. It might be a control cable guide fitting that was originally fastened to a sheet of something like a bulkhead, or floor panel or wing rib/spar, etc. From the size of the holes in the tabs (think partly occluded rivet holes, original size about 3/16") it looks about 4"-5" square. At that scale, the cable size is about right for a control cable.
Second, Directly below that center hole, and apparently in front of the fitting is what looks like a sheave viewed from the side with two arms holding the axle which is 10 oclock - 4 oclock in the pic.
Third, behind the fitting, in a 10 oclock direction and away from the viewer, is what might be a tube with a square cross-section on the outside (see why TIGHAR needs the blueprints?) And at the back of that tube is what might be a circular mount. I can see this fitting bridging a space between two bulkheads, or between two wing sections.
Fourth, while it’s probably nothing at all, just coincidence, the “V” in the cable resting on something could be one of those fittings that allows a change in the direction of a control cable. All this stuff together is certainly man-made. All this stuff together says maybe airplane. But, that’s all it says
Control cable guide sounds good. Which one is anyone’s guess. There could be 20 or 30 such guides in an L-10.
BTW, the control cables were almost certainly steel. Much of the sheaves and yokes and arms were also likely to have been steel. Al is too soft in this usage. And, almost all of that steel would have had corrosion protection because passing between the plane and its control surfaces opens the system to rain. Also, builders would have avoided direct contact between steel and aluminum because of potential galvanic action and associated corrosion -- a second argument for protecting the steel. Which may explain the apparent lack of marine growth on these few parts.
Last, if it's not a CS fitting, it might be a fitting exiting from a fuel tank but I’d expect both ends to be circular to hold gaskets, and I’d expect more than 4 fasteners. So, I'd put this probability at the low end.
Regards to all