Thanks for the info. I think I have a clue what is going on now. I was happy to find one but not happy about rounds getting under the lifter.
I hit the range again today and put 10 strings of 5 shots each through it without any jams with two ammos. 38 Special LRN 150 grain Winchester White Box, and Fiocchi FMJ truncated cone 142 grain.
I grew up instructed to vigorously work a pump, whether an 1890 or a shotgun. And it worked for those other pumps I have used. When you vigorously work the Thunderbolt, rounds in the mag tube can slip out past the cartridge stop and jam under the lifter.
You can work it firm and fast, like I did today. But vigorously beating it like a red headed step child jars it, and that seems to be what allows a round to possibly to jam under the lifter.
The good news is, knowing this you can probably reliably work the gun 99.9% of the time.
The bad news is, in the excitement of competition, let alone getting shot at or having an animal run at you, chances are you are going to pump that rifle like a one-legged madman at an ass kicking contest. That will jar it. Working it in a way that jars it I could get no more than 6 rounds through it without a jam.
The medium news is that it is probably an easy fix. The cartridge stop is not very wide, and does not come up to obscure all that much of the magazine tube opening. Depending on what it is made of, you could probably weld the tip of it, and then file it down so that it is a millimeter or so taller and wider. At least I think so. I have not taken it down enough to see if that will mess with anything. That will make it harder for the round to jump over it under the lifter when the gun is jarred.

The cartridge stop is also ridiculously easy to push down. A jarring of the gun, and the rim of the cartridge in the mag tube, under pressure from the mag tube spring, have it easier than a dude with a cool car and a drunk fat chick for a prom date.
The cartridge stop spring needs more oomph. I don’t now how easy it would be to source a stronger flat, or to tack a small coiled spring under it, but that would also help ameliorate the problem. One or the other or both, but that should solve it.

Anyways, if it was that easy to fix, why has Taurus not done it? Why have the CASS gunsmiths not done it? I don’t know, the other Thunderbolt clones that cost two to three times as much may just be easier to pick up instead of messing with these. They are not easy to find and have been discontinued twice.
I have seen plenty of other stuff on cars, equipment, other firearms that I do not know why the company made it or still makes it that way, etc.