How do I clip with these pedals?
 

I picked up this '85(?) Trek 500 frame from the junkyard. The pedals look like a quill/track pedal according to Wikipedia, but the front (after removing the reflectors) is recessed so I can't attach my toe clips. Are those holes to the rear of the top for an early clipless system? Can the missing part be used with modern shoes?

Odd. It looks as if your pedals have been put on the wrong sides of the bike.

In the second picture of your post, the little triangle is facing toward the front of the bike. Those are usually at the bottom of the pedal (like yours) but on the rear of the pedal.

So, I think your pedals have been mixed up. I don't know what you can do in that case, as the thread are quite possibly toast if try to take the pedals off. Either way, the cages would mount on that bit projecting from the front of the pedal in the fourth picture. I don't know where to get cages with that particular pattern.

Switch shafts, or switch bodies, your choice.

Oops. Yeah... I must have reversed them when redoing the bearings. So an oddball cage, then?

Yeah, I've never seen a cage mounted like that, but then again, I'm generally a n00by to the C&V stuff.

The toe clips mount to those two holes that are pointing up on the pedal. ie: good luck finding some. If you're set on toe clips, I'd just find a new set of pedals.