Tripod pan/tilt head tilt axis won’t lock under load — worn part or missing washer?

I don't know about that model specifically, but that's a pretty typical design for a pan/tilt tripod head. Turning the handle clockwise squeezes the right side of the head around a shaft, increasing friction and eventually locking the tilt mechanism. I doubt that you're missing any part; it's far more likely that some part is worn out. It looks like there's some red plastic or rubber material caught in the area that should compress as the handle is turned.

To attempt a repair, I'd start by gently prying the red "Albinar" medallion out -- it'll probably just pop out. You should then be able to see whether there's some plastic or rubber material that needs to be replaced or whether the metal collar makes direct contact with the shaft or bolt that it compresses. In the latter case, the problem may be that the metal parts are simply worn and you'll need to add some shim material to help increase compression. It looks like the parts are otherwise in decent condition -- the threads on the handle look good, the collar isn't cracked, so there's no reason that you shouldn't be able to restore this head if you want to. A great source for metal shim stock and different types and thicknesses of rubber and plastic is mcmaster.com.

On the other hand, you can buy a brand new pan/tilt head of similar or better quality for around $25. Unless you're really attached to this one in particular, it may not make sense to repair it.