Looks like he mounted an electric pony starter on the fender. If at all possible can you find a serial number amongst the documentation? That will provide a decent amount of information on the unit and would make tracking it down easier, if it's still extant.

No problem, like Rome said, it's a 5J which means it is a 50" gauge (width center to center of the tracks) and has a D3400 diesel engine, dry clutch, and in this case, rope start pony that your Grandad added a farm-built electric starter to which is no problem at all. The alternative gauge was 40" and that variant's serial # started with 3J and then 4 digits. Mine is 3J4086. The J series D2 was the first series, superseded by the U series, which I also have an example of: serial # 5U13753. It also has a seat fuel tank (vs. a side tank that sits on the left fender). No Cat electrics by the looks of it (lights, generator, battery, starter) which were all optional, and we can't see what's on the back - could be a rear mounted hydraulic pump to power the blade rams, a PTO, a winch, or nothing for example. I suspect it might have a front mounted hydraulic pump due to the guard in front of the radiator which is how Cat did it, but of course the guard could be there solely to protect the radiator. Looks like a "custom" seat cushion, out of your old 1973 Datsun : )