https://www.google.co.uk/search?q=Butte ... ent=safari
I do not think Butterworth ever did a Jowett engine from what I can see. However.....
https://forums.autosport.com/topic/1534 ... tterworth/
where PJGD comments
Posted 01 September 2011 - 00:01
Circa 1973, I responded to an advertisement in, I forget now which periodical, for a Butterworth swing-valve engine, and when I went around to the address in the vicinity of Farnborough, Hants, lo-and-behold but it was A J Butterworth himself. I had a good look at the engine for sale, but then he asked what chassis I had in mind for it, and my answer "a Jowett Jupiter chassis", (understandably) did not meet with his expectations for he responded "No, not interested; it would never be competitive". Oh dear. Just as well as I would not have been able to afford it anyway I expect, nor could I have done it justice.
PJGD