I thought I might be so bold as to say that I now have Nš51 at home, after a small pit stop. Poor machine has only seen fresh water but I hope to rectify that this Saturday. She is lovely to look at and a dream in the water. Early days yet but it will be fun to start from scratch again.
Following the machine's policy of 'no tools' I swapped the back plate nuts for these:
They are the same 50mm fit, don't rub the dry suit up so bad and the can be removed by hand. (No I don't work for Portland)
Rgds Fil