J619's, J621's, 623's & 613's
The J619 was not built in any great numbers.
As Des stated it was cobbled together - Cat prime mover & Johnson bowl.
The J621 was the same deal.
Cat had to get into the elevator market as IH, Allis-Chalmers, Michigan & Wabco were already well established there and taking sales away.
Caterpillar bought Johnson outright to secure the elevating scraper technology & save themselves heaps of $$'s in R&D costs (they couldn't buy Hancock because Clark got in first !).
The 623 was the J621's replacement with a few refinements over the J621.
It too was not built in any great quantity.
However, it's replacement, the 623B was a huge success for Caterpillar and also a great machine to operate.
The 613A was designed and built by Johnson who were going to market it themselves up against the Wabco 111A & Euclid S-12E but Cat bought them out before they could market it.
If you have a look at the serial number plates of the early 613's you will see "built by Johnson Manufacturing" on the plate (both tractor & scraper).
ointhead