That is one of the last open class SR390's built by Kawasaki - they built two or maybe three for the 1978 Trans AMA series at the back end of 1978. They were ridden by Jimmy Weinert and Gaylon Mosier. The interesting twist to the story, is that in 1978 Dave Thorpe, at that time a 16 year old schoolboy star in the UK , was riding Hondas. He wanted to switch to the open class in 1979 to ride the British Support Championship. Honda could only offer him a 360 Mugen, and he wanted a full works ride. Team Kawasaki boss Alec Wright wanted him on his team,. so he bought Jimmy Weinerts SR390 over from Kawasaki USA after the Trans AMA series and gave it to Thorpe to ride for the 1979 British Support Championship. Thorpe rode it from January till late April 1979 when his works Unitrak arrived. He won the British Support Championship riding the SR390.
Hope this helps with your project Johnny Orchard.
Regards
Rob.