I believe the 4T model tractors were built with specific US Army requirements (attachments), and 6T tractors were built for the U.S. Navy, and had USN-specified attachments, such as a front mounted winch.
Typical attachments were full lighting and electrical systems, plus military hold-down brackets for items such as jerrycans, shovels, rifle, Alemite track lube greaser, etc.
These tractors were quite often worked "around the clock" (24 hrs a day), especially when constructing critical projects, such as new airfields for Allied Forces. So the lighting systems were an important addition.
As far as lighting systems go, they would be regarded as "pretty rudimentary", by todays operators!
The 4T was equipped as standard for the U.S. Army with the rear-mounted Le Tourneau "Model R" cable control unit, and the LeTourneau Model XD9 straight blade.
Some military D7's were fitted with the LeTourneau WCK7 angle blade instead of the straight blade.
Below is a video of a working D7 4T in excellent original condition. It has the Model R LeTourneau cable control and an XD9 straight blade. LeTourneau called his cable control, a "Power Control Unit".
This D7 4T is near-perfect in its WW2 U.S. Army originality, and the only things that are not genuine original, are the front idlers.
Take a close look at all the U.S. Army attachments that didn't come with a civilian D7.
The tractor is sporting later model disc-type idlers (obviously due to undercarriage replacement), the original idlers were spoked.
The track is fitted with rubber blocks on every second track shoe, so it can be driven on asphalt roads without the grousers tearing up the road surface.
Here is also a good "write-up" on the D7 history.
The story of the early Caterpillar D7 in NZ is also the story of New Zealand logging, roading, quarrying and the Public Works Department.
contractormag.co.nz