Yes Honda CB or CJ 350The donor clutch used is from a 1970's Honda so not suprised they are difficult to find today. Occasionally one will appear on eBay which would almost mean a full time job for somebody at Conways to keep a constant eye out for them. Its not as simple as bolt onto a burman chainwheel as a special adaptor for the center is required which engages with the mainshaft. This adaptor uses a lot more of the mainshaft spline which is good as the original Burman adaptor only just engaged.
When I wanted a Honda / Burman clutch conversion I found and bought the Honda clutch myself and then sent it to Conways to be adapted etc.
I seem to remember its a Honda CJ360 clutch from mid 1970's but you may want to get that confirmed by Conways.
A parallel twin I beleive