Locked Up Again, no apparent reason.

[toppop52]

I promised Boomer updates as they happened, so here is a rundown;
Got the float sealed up with a super thin layer of fuel tank sealant and the tractor runs great, after running a while we got the carb adjustments close. I worked it for about 1.5-2 hours today, with a lunch break in the middle. On the second time out the choke cable slipped in it's bracket and I had to stop and replace a stripped bolt to fix that. Upon attempting to re-start the engine it was just like before we took it off and pulled it apart last Sunday, locked tight! It did that when I took it off the trailer a couple of weeks ago, ran fine coming off the trailer, shut it down and on re-start, locked up. The starter drive releases if you tap it with a hammer so it's not that. It's a multi range hydrostatic so I don't see how that could do it. It keeps pointing back to the engine but we had it apart and there was no noticeable wear, all bearings were free and clearances were good. I'm absolutely lost!