Absolutely spot on Pete many thanks, thats a long outstanding mystery solved.
Not that Ive worried about it for that long but every now and then I come across it pick it up wind the handle in and out perhaps take the threaded part off the end look and say to myself well its good quality engineering well made -- put it back again and think well I dunno and carry on - all's well with life.
I never thought to place it with the lathe before, I have now, the stem fits my lathe banjo assembly my chuck fits onto the threaded part - its the same thread as my headstock spindle. Now that this has all been revealed I do vaguely remember that the previous owner made children's toys and threaded boxes so this is what he used, I don't recall any cross slide in the collection of things that were being sold but maybe there was. The stem part that fits into the banjo has a loose section of copper pipe that fits over the stem and leaves about 21/2" uncovered. I suspect that when the stem is slotted into the banjo the copper pipe was a depth gauge or height stop so that the height at where the cutter was held would be constant and not reliant on the user getting the height right when putting the tool into the banjo.
I've looked at the links you sent so I see how it works, I've attached another photo with the handle wound outwards. The round disk next to the handle is free to rotate, (but fixed in position ie it cant slide along the thread towards the body) there are 2 holes in the disk but no holes in the main body. Perhaps the space between the disk and the body had a spacer inserted to that when the handle is wound inwards and the cutter (on the other end) moves into the workpiece it stops at a depth selected by the thickness of the inserted spacer (and cuts the thread to the depth you need) Perhaps the 2 holes were intend for a spacer to be clipped into or screwed into?
The only thing thats not clear is how the thread is cut, yes I can see how the cutter cuts and you wind the handle to move the cutter and form the thread but why doesn't it just act as a gouge and just cut the wood away instead of forming the thread. I will need to think on this and watch a few more YouTubes until its clear.
thanks again
Paul