Caterpillar introduced the first Caterpillar D9 (D) in 1954 starting a serie which lead from D9E, D9G, D9H, D9L, D9N, D9R to todays latest Caterpillar D9T.
If you want to know all about the fascinating world of the D9 prototype from 1946 until production of the first machine and the development and history of the model to the Caterpillar D9R then I can highly recommend the book "Caterpillar D9 D9R" writen by Thomas Wilk and Urs Peyer.
Of the D9 series Track-Type Tractor are 1:50 scale models of the D9E made by RR Models and D9G of Gesha, Conrad, Norscot and EMD. The D9N and D9R of NZG. In 1:48 scale by CCM are severall versions of the D9G, D9H, D9L and D9T as models released. Besides the standard versions, there are also a number of special versions appeared as the "DD"(QUAD) and "SxS"(Side by Side).