I'm not familiar with that particular label, but I often find that vintage garments have numbers, words and all kinds of things written in them for all kinds of reasons, so I wouldn't take it that it was necessarily written in there at the time of manufacture, eg, it could have been written in there by a collector or a wardrobe department, for identification and catalogueing.
My experience is that bespoke garments often have specific labels that might also include info like dates, customer name etc.
Curious to see what others think?
Nicole