What kind of information are you looking for? Something simple such as year of build, etc., - or maintenance information, and major repair information?
78U0986 is an early model 931. It was built in 1974, the 931 and D3 were built from 1973.
It has a precombustion chamber design of fuel injection, in the late 1970's Cat went over to Direct Injection for the 3204.
The 3204 is one-half of a 3208 V8, so there's a large amount of interchangeability with 3208 components.
If you go looking for parts, use 78U00986 as the Serial #, because Cat part systems and many others parts systems operate on 5 digit Serial #'s with the prefix.
The D3, 931 and 910 loader were all built on the same design plans, with an 80% commonality of parts and components.
Initially, only a 3F/1R transmission was available, within a couple of years a 3F/3R transmission became available as an option.
78U identifies your loader as a 3F/1R transmission. If you had a 3F/3R transmission, the Serial # prefix would be different.
Early D3's, 931's and 910's had ROPS canopies that were only 2-post design. Within 2 or 3 years of release, the ROPS design was changed to a more robust 4-post design.
All D3's, 931's and 910's were built in Japan by Mitsubishi as part of a Caterpillar/Mitsubishi joint venture.
They were built in an under-utilised Mitsubishi factory and you'll find the Mitsubishi "3 Diamonds" logo on many castings.
Below is a link to an interesting article about the engineering behind these machines. It was written by a Cat engineer, Richard K. Liess, who was in charge of design and development of the D3, 931 and 910 product line.