Jonathan Greene expanded from his original note: All threads are in good condition. All brands of derailleurs fit just fine. This one just does not thread more than 2-3 turns.
My Rally bolt has only 3 to 4-threads (turns) max. The shaft extends approximately 6-MM beyond the shoulder, with approximately 4-MM of proud treading. Has the 2-MM between the treads and the shoulder been properly reduced, or does it restrict full insertion? That's what it seems to be happening. I'm assuming you have already done the typical things like removing any excessive paint at the bearing surface. Perhaps if you have a lot of patience and a very little file you can convince it to fit properly. I think my approach would be to temporairly swap bolts -- if the new bolt fits properly, then try to determine what's different.
Maitland FL