GW = "Equipment Truck", but the primary focus was and still is on rescue equipment. There are three levels of specification (GW 1, 2 and 3) with increasingly large and comprehensive equipment loads. This 12 ton truck will have been a GW2. The GW1 was usually an eight ton truck, while the GW3 was a 16 or even 19 ton chassis. Most GW had standard truck cabs, but a few crew cabs were in use, and an example of a GW3 on 19 ton chassis with a six seat cab can be found in photo 42499 on this site.
Added by Rob Johnson on 08 April 2021.