Sorry forgot to mention why these were so popular. Ordinary sprockets could be used, you could go down to a 12T on these. No side-play or float which was very common on the blocks available back then. Easy to change ratios and larger bearings than freewheel blocks. The main thing against them was they were expensive. Bleeding marvellous for tandems the best ever.
Thats all for now. Keep those wheels spinning, in your memories if not still on the road. Be lucky Mick Butler Huntingdon UK.