SDMB fail me not! What screw is this? I have a pocket watch pouch and the flap is held on by a bolt which has this screw at the end. The screw keeps coming loose and falling off. So I need to have some spares. The head is 1 cm in diameter, and the diameter of the thread appears to be 3mm. The length appears to be 6mm. These are measurements eyeballed against the tape in the picture, so are only rough - I don’t have a micrometer.
Application of Loctite is likely to be more effective than periodically replacing specialized fasteners.
Does not the discolouring on the screw indicate that some form of glue has already been applied and failed?
More to the point, I’ve already lost one and need a replacement.
Possibly.
But the proper flavor of Loctite, properly applied, will without question make that screw stay in place.
It’s custom.
You are never going to find an exact replacement by looking though an industrial supply catalog like McMaster-Carr.You might have some luck contacting the watch manufacturer or repair place.
But, Loctite is the best solution…
Were you in my city, I would recommend a company named “Star Nut and Bolt”. Have you a company like that nearby?
That I very much doubt. How can you tell?
We have Screwfix here and they don’t know.
Why do you doubt that? The bevel on the head and the non-typically huge ratio of head diameter to thread diameter fairly scream “specialized design” to me. My perspective is from 40 years of dealing with parallel situations in auto repair, running across custom hardware that is available only from the dealer, if at all. Do you have some experience to the contrary?
Doesn’t that tell you something?
Yup. 
You might try local jewelers/watchmakers. I’ve seen that type of screw before, but it’s basically just a specially made cap screw, similar to this.
If I read right this is off of the watch pouch, not the watch itself. It looks like half of a “Chicago Screw” or the back screw of a concho. You might check someplace that sells leather craft supplies.
You could probably find a similar button head cap screw with the correct length and thread size. The profile of the button head will probably be higher than the flat head of your screw but I don’t know if that matters.
Bingo! There are plenty available on the usual online retailers. There’s even a specialist supplier.
Half a Chicago screw … like several posts above said, that’s called a cap screw.
Now half a pair a scissors is a …
A new screw may not help. The internal threads in the flap could be stripped. If you don’t want to go with glue you might consider a rivet instead. Possibly the female half of the screw set used to be there and went missing.
Watches … . Its a type of screw used in jewelry and watches and the like…
Jewellers have these, if you lose it you can drop into a watch repairer and … one minute later, fixed… They may even have a tool to fix the threads…
You can use loctite or plumbers tape… hang on, you can use any plastic sheeting, like a shopping bag or something, instead of plumbers tape, as you don’t care about water tightness.
That is not a cap screw. It is a flat head machine screw, sometimes called a cheese head machine screw. All you need to know is the length and diameter of the shaft, how many threads per inch (or metric equivalent).
I would call it a slotted pan head machine screw. Go down to a hardware store and find the correct threaded bolt that fits so you know what size and thread count you’re looking for.
It’s probably going to be trivial to find a screw that matches the thread.
Finding one with the exact same head is the hard part. If you are willing to get something close, then take it down to your nearest Ace hardware and see what they have that is similar.