Avro 504J

The Avro 504J appeared first in autumn 1916 and this new version was powered by a 100hp Gnome Monosoupape engine. The engine cowling was slightly modified in order to make easier to fit other type of engines.
It was usually called Mono-Avro in the RFC, to which it was destined. It had therefore a comma shaped vertical stabilizer and shorter back ailerons.
As the engines were harder to manufacture, the airplanes were completed as those were completed, that's why many of them ended up being completed as the K model.
It was destined from the very beginning to the training role and it was easy to fly and powerful for it's era. That's why it was the basis for a new training method used by the RAF until the end of the World War II. However, at the end of 1916, Avro was commanded to concentrate their manufacturing efforts on the BE.2c instead of the Avro 504. However, a bit later, the 504J was chosen as the standard RFC trainer and the production was resumed.
It wasn't used only by the United Kingdom but also by the American Expeditionary Forces and also by the Chilean army which, fearing a war against it's neighbours Argentina and Bolivia, rearmed themselves.

Sources:
1. https://fr.wikipedia.org/wiki/Avro_504 (translated)
2. Salamander Books - The Complete Book of Fighters