Odd crankarm problem

My daily beater is sick. The bike is a 1970's/early '80's (probably) issue KHS 10-speed, very confortable, perfect size and geometry for commuting, so although the bike may be a cheapie, I'd like to keep it around. The cranks are slightly unusual insofar as they are held in place by cotter pins and they have a little ridge inside them, which slides into as keyway in the bottom bracket axle. The guy at the bike shop worth going to for vintage bikes says he's never seen one like that. The left crankarm has lost said ridge - eaten away by decades of being cranked, no doubt - and CANNOT be tightened by cotter pin alone. No matter how far you drive the pin in, no matter how tightly you screw it shut, it wiggles loose almost instantly. The right crankarm is fine, solid as a rock. The aforementioned guy at the bike shop says that's impossible, cotter pin should be enough. It's not. I've finally given up trying to fix what I have - too many failed attempts in too many different ways - and resigned myself to the fact that I will probably have to replace the entire bottom bracket plus crankarms, hoping I can find one that fits... This bike did seem to come with a host of components no one has ever heard of, the shifting system was rather unusual, too, didn't even bother trying to find replacement parts, just changed the whole thing. Anyone have any better ideas?