The Holt 75 and the smaller 45 were agricultural tractors. The 75 was adapted for service as the 15-ton artillery tractor and the 6 cylinder version as the 20-ton artillery tractor.
The US Army at the time rated the tractors by drawbar pull so the tonnage is not related to vehicle weight.
The main (by numbers) artillery tractors in WW1 were the 5-ton and 10-ton which were loosely based on the Holt 45 tractor but were quite different vehicles compared to the agricultural tractors.
The full manual for the 75 is at:
archive.org/details/careandoperatio00incogoog
For the 45 at:
https://babel.hathitrust.org/cgi/pt?...39015075038789
5-ton artillery tractor
radionerds.com/index.php/File:Handbook_5-Ton_Artillery_Tractor_Form_no.1996.pdf
10-ton artillery tractor
https://www.radionerds.com/index.php...TON_MODEL_1917
There also was a 2.5-ton artillery tractor designed for towing light field artillery. However this used a suspension copied from the Renault FT tank and wasn't very successful - relatively few were built and I don't think there are any surviving complete examples today.
There are a fair number of surviving 5- and 10-ton artillery tractors. Although they were geared too high to be useful agricultural tractors many saw long service in forestry and construction after being sold by the US Army in the 1920s.