Found this in my work truck and I'm drawing a blank - what is it?

Sounds a lot like an old thermal fuse link, probably off a water heater, furnace, or some kind of heating appliance. Those are ceramic tubes with metal caps and a single pigtail wire, and they were common on older equipment as a safety cutoff. If the temp inside the appliance got too high, the link would open and kill the circuit. The 20A stamp would fit too, since they were rated for a current limit along with a temperature rating that's usually faded off first. I've pulled a few of these out of older water heaters and garage heaters over the years. They always end up in the bottom of the van because you replace one, toss the part in the drawer, and forget about it. The chunky shape throws people off because they expect a fuse to look like the little glass ones or the cartridge style that goes in a fuse holder. The pigtail is the giveaway. Cartridge fuses for panels don't have leads, they just make contact end to end. Easy way to confirm it: put a meter across the two metal caps and check for continuity. If it's open, it did its job at some point and that's probably why it came out of whatever it was in. If it's still closed, it was likely just a spare that never got used.

Either way it's a one-shot part. Once it opens you replace it, you don't reset it. And since the markings are gone you've got no way to know the temperature rating anymore, so I wouldn't put it back into service on anything even if it tests fine. It's a junk bin item at this point, but at least you can stop wondering about it. My money's on it came off a job where someone swapped out an old limit control or water heater thermostat and the old link rolled into the drawer.

Yep, that's a thermal cutoff alright. I've been pulling those out of dryers and old wall heaters since the 80s. Back then we'd just clip a jumper across the two terminals to get the machine running for a test, then tell the customer to order the right replacement. Wouldn't do that today, but it got the job done plenty of times.

The reason your stamp is faded is those things live next to the heat source, so the printing cooks off long before the part fails. I'd bet the temperture rating wore away years before you ever saw it. Toss it in the bin with the rest of the mysteries, every van I've owned had a drawer full of those.

Those things end up in every van drawer eventually. I had one rattling around for a good year before I figured out it came off a baseboard heater swap. Trick I learned from the old timer I used to ride with: give it a shake near your ear. If the guts melted you can sometimes hear a little rattle inside the ceramic. Not foolproof but it settles the mystery fast when it works. If it does test dead, just toss it. I kept mine on the shelf for months thinking I'd match it up at the supply house someday, never happened once the markings were gone.

Update: mystery solved. Took the second reply's advice and traced it back through my job log from two summers back. Turns out it came off an old wall furnace I replaced, so the heating appliance guess was spot on. Did the ear shake test too and heard a faint rattle, so the guts did their job and melted. Tossed it in the scrap bin, finally sleeping better.