[grumpy]

Turning to that well-illustrated site to identify a cap c. 1890 [it was Norfolk Regiment] I noticed the following:

WHO WORE THE CAPS?

All the officers of the regiment including the Quartermaster.
The Battalion HQ warrant officers and staff sergeants. This included the following appointments; The Sergeant Major, the Bandmaster, the Schoolmaster and the Quarter-Master Sergeant. Also the Musketry Sergeant Instructor, Gymnastics Sergeant Instructor, Band Sergeant, Sergeant Drummer (Later Drum Major), the Armourer Sergeant, Orderly Room Sergeant, Pioneer Sergeant, Signals Sergeant, Hospital Sergeant, Sergeant Master Cook, Sergeant Master Tailor, Sergeant Master Shoemaker

This is a very long list, 16 other ranks in all, and yet one rarely sees more than about eight such caps in period portraits of "The Warrant Officers and Sergeants".
I venture to suggest that the cap was only worn by those entitled to First Class Clothing, as defined, for example, in Clothing Regs 1881, 1882, 1887, 1894 and 1914. That list is usually of about eight appointments for an infantry battalion, and indeed about eight such caps are generally seen, in the front and second row. These are not all "First Class Staff" e.g. band sergeant and sergeant drummer, but the appointment carries the tunic quality, cap, and sword to go with it.
I would be very interested to see photographs of the following wearing, in such posed groups, the splendid cap:
pioneer sergeant, signaller sergeant, master cook, master tailor and master shoemaker.

Two other minor points: the armourer rarely [but not never] seems to appear in the [infantry] cap because he is "with" the regiment, not "of" it; and the change drum major/ sergeant drummer was that way round in historical terms, although it probably reverted after 1920, when I lose interest.

Examples to refute the above will be most welcome, as ever, uniform being uniform.

Any mistakes are mine