Just to confirm Diane – I made mine from original Helix Engineering castings and drawings. I gave the drawings, which incidentally were the very best 'model engineering' drawings I've used, to Jo Thoms. Maybe she still has them?
I believe at the time, the father and son team (the name escapes me but it may have been Buckley) who produced them under the Helix name were active members of the Bolton museum.
Hope that helps some – Ramon