Matt Veerkamp loves Caterpillar brand crawler tractors, and restores them. He found the first Caterpillar brand crawler tractor ever built (1926 Caterpillar Twenty) about 14 years ago, and now, he's restored a 1929 Fifteen, which he found on Craigslist. It's the expo model for the Caterpillar Fifteen, the only one known to exist.

When he got the Fifteen home to the shop, he discovered some other unique things about it. Some of the parts, including the axles and gears, were from Caterpillar’s Model Ten. 

 “The production model Cat Fifteen was different in a lot of ways than this one.” Veerkamp determined he had the prototype or concept Fifteen. 

“It was a bigger version of the Cat 10, which came out the year previous. And I think they ran it, figured out what didn't work and then made a production model after that.”

https://www.equipmentworld.com/collectors-corner/article/15287217/caterpillars-rare-1929-expo-fifteen-turns-up-on-craigslist