Is that part visible from the outside or do you need to take the toolpost apart to see it? I have a 7x10 lathe (which should have the same part), but I'm not sure that I want to take the toolpost apart right now, and I'm too lazy to walk down to the basement to compare the exploded part diagram with the lathe.
Apparently you can order the part from Grizzly for $2.50 (if I have read it correctly):
P8688109 POSITIONING PIN
based on the parts list at http://www.grizzly.com/products/g8688/parts