I'd almost bet that screw is long gone out the stack. I think I see a consensus forming that the most it might have done is bend a valve -- not a disaster, but not exactly a minor thing to fix.
If it is still in there, and as to your question of which cylinder to look in -- you're going to do a compresion check anyway, so you're going to have all the plugs out. Couldn't hurt to have a peek in each one, but do the compression test, and look for the screw first in any cylinder that comes up with low compression. Turn the engine by hand until that piston comes up to the top. If the screw is still in there, but you can't see it, try blowing a little compressed air in through the plug hole and see if you can blow it around to where you can see it. Then you can get it out with a magnet or a dab of rease on a stick, Worst case is you'll have to pull the head to fix the valve and you can take the screw out then, if it's still there.
That's all best case. Worse relates to what somebody mentioned up above about possibly a gear tooth chipping off and jamming, causing you to stop so abruptly. My neighbor lost a tooth on the pinion in the rear end of his pickup last summer. That's a lot tighter quarters than the tranny and diff on your H. Still, that broken tooth got caught up and grenaded the rest of the pinion. It bound up so tight that it snapped his rear driveshaft. Anyway, after we got all that fixed, he ran it only a short time before he got a low-end knock in the engine. We opened up the bottom and found the rod bearing on #1 had spun and #2 was about ready to go. 3 through 6 looked okay, but 7 and 8 were messy, too. Bottom line, the abrupt stop bent his crank. I hope that isn't the source of your knock and vibration, but it's something to keep in mind.
If it were mine, I'd check out the top end first. Fix anything there and then, if the vibe and knock are still there, drop the pan and have a look at the bearings in the lower end. If the crank was damaged in the stop, then it's a whole other can of worms about what to do.
If the latter is the case (and I hope it's not) and you get it fixed, you'll likely want to lift the deck and look your gears over. Sometimes those broken gear teeth will sit harmlessly in the bottom of the gearcase, other times they'll get swept back up and do more damage the next time they get caught.
Sorry about your troubles, and I hope the problem turns out to be something simple. Keep us posted.
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Today's Featured Article - An Old-Time Tractor Demonstration - by Kim Pratt. Sam was born in rural Kansas in 1926. His dad was a hard-working farmer and the children worked hard everyday to help ends meet. In the rural area he grew up in, the highlight of the week was Saturday when many people took a break from their work to go to town. It was on one such Saturday in the early 1940's when Sam was 16 years old that he ended up in Dennison, Kansas to watch a demonstration of a new tractor being put on by a local dealer. It was an Allis-Chalmers tractor dealership,
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