Vg,

If the wiper arm has come off of the center "drive" post, as I believe you have stated, you probably are looking at a retainer clip that has rusted and fallen away. If you can't feel it at the top of the mounting post, then it fell off and you will need to replace it. If the arm came off of the post, it should not be there.

If it fell away, it should be laying on your floorboard, or in the folds of you carpet or firewall liner, or even right below the mounting pin, still up in there in the wiper hardware, but it may not be in one piece, if rust is what caused it to fracture and fall away.

It's not a circlip, but a steel clip, bent in two so that it has two "legs", one is flat and notched and slips into a groove in the mounting pin.

The other leg is slightly curved and slips over the top end of the mounting pin, I assume to help as a retainer.

You can mount it from below, on your back under the dash. It's a challenge, but I've done it, with just the right, very-long, curved, needle nose pliars.

gary - '67 122S, '72 ES