Holt Farm tractors by series

In the late 1800s, Benjamin Holt began manufacturing self-levelling combine for use on hillsides in the western United States. Holt began building steam tractors and began experimenting with crawler tracks. The first tracked Holt was tested in 1904, and a commentator is reported to have said, "She crawls along like a caterpillar", providing the inspiration for the trademark. In 1925, Holt merged with rival Best to form the Caterpillar Tractor Company.

Caterpillar - official site
ACMOC - Antique Caterpillar Machinery Owners Club
Antique Caterpillar Machinery Enthusiasts - Cat forum