Those cases that were round on the back (with hole) were special, made for RC cases. Most of these engines had a spring starter in that rear housing. These were very long lived engines, had good power and are well mannered. Only thing I can say bad about them is that they shake a bit but not excessive by any means. These seem quite happy on 24-10 props. My guess is that engine is a 75 as there seemed to be far more 75s out there than 65s. These make great warbird engines.