The 977D was introduced in early 1956, was powered by the D318 engine rated at 100HP and used D6 undercarriage.
It was fitted with a 2.25 cu yd bucket, had oil master clutch as standard fit, and weighed 31,795lbs.
This traxcavator started with a serial number prefix of 20A, and ran from 1956 to 1958 with S/No's from 20A0001 to 20A2508.
In 1958, the upgraded 977E was introduced. The 977E was fitted with D7 undercarriage and specs were relatively unchanged from the 977D, apart from gaining weight - to 34,740lbs with the heavier undercarriage.
Serial numbers of the 977E, built from 1958 to 1960, ran from 20A2509 to 20A4222.
Both the 977D and 977E were gear (direct) drive, with hand clutch and sliding gear transmission.
In 1960, the "new" 997H was released, this traxcavator was now fitted with the turbocharged D333 engine, was 150HP, came with a powershift transmission, oil-cooled steering clutches, and weighed 36,600lbs.
Serial numbers of the 977H were from 53A0001 to 53A8886. 977H production ended in 1966.
I see nothing about differences in final drives between the models, but I may be missing something in the design changes.
Not sure if the traxcavator in your photos is a 977E or 977H, the transmission type will be the pointer.
That tractor has had a long hard life, and is not worth much if it doesn't steer. Those track pads are worn right out and you'll be struggling for traction with them.
Steering controls are simple levers operating a small hydraulic spool to disengage the steering clutches.