I'll attempt to address all your very helpful points :
1 RH prop, RH engine. so box must be LH
2 separate throttle and F-N-R cockpit
3 all my observations refer to the twiddly lillte lever on the box
4 single helm
5 unit came combined off another boat. i can only assume it was never gievne the reindexation it needed, or the prior boat had LH prop. it was a single
6 it actually works and worked fine, but because the Reverse clutch
is for occasional use it is smaller and overheats / slips after 6 or so hours of driving the boat forward!!!
I shall now look at Gord Mays links !
many thanks for all your inputs folks