Featured here is a photo of a dozer sent to of an old Cat in a shed in Idaho. There is some discussion about whether it is properly called an RD-6 or simply a D-6. Call it what you may. Introduced in 1935 as a more powerful replacement for the Cat 40, Cat manufactured the serial number family called the "2H" series for the first 2 digits of the serial number. These were made until about 1941, when the more familiar D6 with a 6 cylinder engine was introduced.
In 1938 Cat began calling the product a "D6" instead of an RD6. There was no change other than the name. This machine is believed to be a 1938 model but bears the moniker RD-6 on the radiator. In any event it featured a 3 cylinder diesel engine dubbed the D6600 for having approximaly 660 Cu in. of displacement based on 3 cylinders of 5 1/4 x 8. Cat built a 4 cylinder version of this same engine for use in the D7 and a 6 cylinder version for use in the D8. Top RPM was around 900, and the horsepower rating on the belt was 51.85 hp. It's predecessor the 40 used a similar 3 cylinder engine with slightly less displacement and power. The 1930's were really the fledgling years of the modern diesel engine.
The origin of Cat was in producing low ground pressure farm tractors for use in the soft ground of the Sacramento Valley, and idea which was quickly adopted and adapted to the steep hills of the Palouse in Washington and Idaho, as well as other areas, not because the ground was soft but because they had the stability and power to work on steep ground.
As you can see a hydraulic system is sort of an after thought, and the blade had to be attached to the swing frames because cat used an integrated design where there was no frame other than the structure of the castings. The engine was supported by a cross spring running from track frame to track frame which could rock up and down allowing both tracks to stay on the ground even if it was uneven. In modern dozers it is usually a steel bar called an 'equalizer bar'. The spring sat in a saddle on the cast iron oilpan and was bolted to the engine block. The radiator in turn was cast iron and bolted to the engine. At this time Caterpillar did not even manufacture blades for the machines. LeTourneau made some to fit under Contract with Cat, but many blades were locally fabricated by local machine shops as an after market item.
Special thanks to the owner for providing me this photo in 2009.