If you haven’t already checked it out, take a look at the other “Long Tom” thread on here that is closely related to your question:
Mack NO6** Artillery tractor used for towing the 155mm Long Tom and the 8 inch guns.
Link:
** The “NO6” designation does NOT mean Mack #6. The NO designation is Mack’s builder factory ID for this vehicle and this is the sixth such design offered to the US Military - the only one accepted by the Army. (The “NO” is just like the GMC Deuce carries a manufacture’s factory designation of CCKW.)