A vintage replacement shirt collar, made by Viyella for their "Clydella" and "Clydellon" boys' school shirts.
These are not "separate" collars, but were intended to be sewn onto the shirt as replacements, once the original collars were worn out, or once the child had started to outgrow the shirt.
The instructions suggest that the replacement collar should be larger than the original one.
Presumably made in the 1950s or 1960s.
These collars were unsold shop stock, and are still marked with the original, pre-decimal, price of 3/11 (three shillings and elevenpence).
Choice of size (this is the neck size in inches)