Thank you Adam and John for your offers of help – much appreciated. I think I need to be self-sufficient, however, in case I need more, sporadically, in the future.
The Westfield site is most interesting (I hadn't found it), particularly the warning that nominal and actual sizes may be quite different. Perhaps that goes some way to answer Jason's points. However I'm not convinced that the Italians in the 1950-60s would have used woodscrews conforming to the gauge number 'standard', and the sizes used in this box seem to bear that out. For all I know, there was a little factory somewhere, churning out 'funnies' for the accordion industry. That industry is interesting: it seems that there were numerous factories (if you can call them that), in two small towns that were, in effect, the accordion manufactory of Italy. Many firms were clearly started by members of the same family. Who made what, or bought in what, and from whom would be virtually impossible to discover. A few videos can be found of the things being made today – mostly hand-work, with little sophistication visible in some firms. The engineering is crude, the material selection questionable, but craftsmen and women triumph over the limitations of their work practice to achieve something beautiful, magical and of considerable complexity.