Karl, for your serial number look near the right-side wheel; it's probably painted over. The serial number will be something like 803 92 4

My 1986 Horse tine attachments are secured with 5 3/4" long 1/2"-20 hex head bolts. The bolt goes through a 1/2" lock washer, then a 1 3/4" flat washer, then into the tine holder. If you've got a 1/2"-20 bolt around, try it in the threads to see if that's the right size, then check your local hardware store for the proper length. I have no idea what the half-moon things are; they're not on my '86 model, and don't show up in the parts catalog.

Key-woodruff is right name for halfmoon pieces. most shops has assortment box of these keys you have to take shaft with you find right one. this want help much but part number on key is 1104436

Here is a link that might be useful: its in this link someplace

Some of the earlier models had a stud w/ 1/2 threaded rod threaded on both ends. that threaded into the tine shaft ,and then the tine holder was placed over the tine shaft which lined up with a woodruff key ....so it's a botled tapered,keyed fit ,once on they very seldom fell off,parts should be available for this unit ,

Now is the time to make sure your tine shaft and seals are not weeping oil either .....it's easier to replace them now,then waiting until all repairs are done ,and you find the seals are no good....my 2 pennies,Jim