That one might be one of the installers numbers - like the headliner story we got from the San Jose worker where he put his number on the seat riser
Agree that the "C4" was likely a "64" or "04"
Looked through my pictures and have a 9/66 San Jose fuse box with "38" written in white on the passenger side of the fuse box. In 67 SJ the seat cushions, top of trans or back side of door panels are a good place to compare numbers
Going on this scenario, which is probably closer to correct, I could interpret the character as a very lazy "6" with a closed loop on the "6", making it a "64" marking.
Looking on the backsides of original door panels, at some other markings found under engine bay paint, checking the seat risers, trans tunnel and the likes for numbers or markings elsewhere, nothing else so far has been noted along the way with the same marking. Under hood locations & the likes would be long gone with a repaint but if this IS my rotation number, it would be nice to know and basically why I asked about the marking in the first place. It couldn't possibly be unique. With data supplied so far by other examples and that I have yet to find this number elsewhere, I am inclined to believe the marking as an "Installer ID".
Just another one of those assemblyline
mysteries until more is revealed I suppose.
Maybe this worker was left-handed, marking is on the LEFT side of the fuse panel