Hi Muzza,
Deas is correct - you have the later series, and I'd guess yours is a '37 or '38 model. I have 1J3796W back in N.Z. and that's a '38 (W stands for wide gauge - the later tractors like the D2 used a different serial number prefix to indicate gauge e.g. D2-4U standard, D2-5U wide).
Shares a some of the undercarriage with the D2. Doesn't have any real oil filtering so a) check the sample, and b) change it often - I think the manual says every 50 hours (weekly if it was busy working every day). Many built and many still around. Get the parts manual and operator's manual from Cat Legendary Literature - there is no service manual but the 8C 20 service manual covers most of the drivetrain I believe. The SP in yours indicates Special Parts, added on the assembly line (as opposed to optional extras such as PTOs added at the Dealer) - I think the most common SP added was altered gearing.