5U is the last series of that model, and the most current design, after 13000 something, and they went to 18000 something in 1957, and that was it for the D2. 5U is a wide gauge, 4U is a narrow gauge, which is the dimension from center of track to center of track.
Steering clutches being free is a bonus on those, they can seize from lack of use, moisture etc., if the tracks are decent, even if not, a complete, later model 5U like that ought to have some interest, even at scrap price, try the ACMOC and ACME antique caterpillar forums, join if need be to get the tractor posted for sale, they are not rare but the size makes them collectible, and maybe it has a dozer blade on it, would be awesome if its a tool bar set up, you can swing those arms back and use a variety of tools, which are not often found, but a tractor set up with that particular dozer/tool bar kit is a useful can collectible feature, maybe you can post a photo of it here, and great job on keeping it from being scrapped, parts alone someone could use, might be a complete salvageable tractor.
Undercarriage replacement parts are hard to find now, no new aftermarket, but you can convert to later D3 worst case scenario.
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