I had the same issue today, but I think the pin came off over the 'head' rather than losing the lynch pin. It's on a Howse rotary cutter and they have the 'slot' at the top of the center post. I'm using TSC vintage pins that look like this:
Problem is the flange on the head is so thin i think it just pulled right through the slot. I ended up slightly damaging the cutter because i raised the hitch, but without the top link, it just had the effect of raising the front while the tail wheel was still on the ground. The leading edge of the deck ran into the PTO shaft and rubbed right through the plastic cover.
So i temporarily replaced it with one of these:
using a 1/4" lynch pin instead of the clip. (It's the same thickness, despite the picture)
Any thoughts? Why is the flange on the 'official' top link pin so thin?