Avro 504K Mk.II

The Avro 504K Mk.II was initially an improved version of the 504K not intended for the military use. In fact it was just an hybrid civilian trainer built in the United Kingdom and Canada consisting on the regular 504K fuselage with the 504N undercarriage and wings. It was powered by the original Gnome engine and made it's maiden flight some time after 1924.

The initial idea of making an hybrid version between the two versions, was to create a cheaper version to be sold to the civilian market, however, it failed and only a few of them were sold. Interesantly enough, Mexico, wanting to replace their aging 504K, bought the manufacturing license of this airplane as they couldn't afford the, by then, expensive 504N. They were locally built in the Talleres Nacionales de Construcción Aeronáutica (TNCA - National Aeronautical Building Workshops) and were used as improvised light bombers -with small bomb boxes attached to the sides- in the Mexican Cristero war by the government forces during 1926-1929.

Sources:
1. http://www.gob.mx/sedena/documentos/material-aereo-historico-de-la-f-a-m (translated)
2. https://en.wikipedia.org/wiki/Avro_504#Variants
3. http://www.mexicanaviationhistory.com/articulos/articulo.php?id=36 (translated)