I have a Boosey and Hawkes instrument that came to me without a screw in its lyre box. Thinking that it might be handy to have a functioning box I contacted a couple of spares suppliers who assured me that their part would fit and work in my B&H. The supplied parts didn't fit so I measured a couple of things (thread diameter and pitch) and used the information to identify the thread type used. I now have a (roughly 1/2" long) 4 BA screw in the Lyre box (sourced it off of eBay), I couldn't get a knurled head head Screw but plain slotted will work for me - assisted by some small handy tool, be careful not to over tighten. The British Association or BA standard for threaded parts is something that was commonly used in the past, it is no longer in common use - expect older instruments to have Imperial (inch based) rather than Metric (millimetre based) threads on them. I hope that the 4BA size details above are a help to someone who finds themself in a similar situation and, of course, that this post actually shows up when someone searches for the deatails in a few years time. What worked for me might not work for someone else, but it's a start.