South Staffs cap badge

[Gary Jucha]

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[leigh kitchen]

The design of badge was in use for over 50 years, so if it's a serial number it could be from an era not covered by the book.
It'd take a bit of effort to stamp a serial number into a slider, but it's not beyond the realms of possibilty - blokes in workshops & armouries for example may be particularly inclined to do this as they would have easier access to the punches than a squaddie in a rifle company.
Numbers or name were sometimes scratched onto the slider, numbers often abbreviated to the last 3 or 4 digits.
Distinguishing marks are often used on army kit - a scratch or dab of paint, that sort of thing so that if you "lose" an item you can try to seek it out amongst your mates kit, but you don't want your name on it because if you nick one off someone else & then your named item turns up you could be in the ****.