The MAFVA magazine "Tankette" had plans and some info in the issue 17/2. They will need to be re-scaled as they were not published to the stated scale. Use a multiplier of 1.80 to take them to 1/35 scale. I have recently finished converting a Centurion to one, just need to paint it. This was a prototype only for elements of the Chieftain and would not have gone into production. However the Centurion X turret, which was similar, was proposed for use on Centurion MK 7 hulls. The gun used for both was the 20pdr type B with fume extractor, but I suppose could have been up-gunned with the 105mm L7. Mark