A vintage mystery

I.m always on the look out for vintage knit-abilia (I thin I just made that word up - someone correct me!!) so yesterday I was delighted with these two finds in Oban's Oxfam shop

A set of long Aero dpns, size 11 (3mm) with the original wrapper marked with the price- 1/6 (thats about 7and a half pence!)  and a knitting needle guage. I already have one of these bell-shaped guages but iIthought this one was rather cute.
It wasn't till I got it home that I realised that the numbers depicting the sizes are a bit strange.

For example, the old size 11 (3mm) needles fit the hole with the number 18 beside it.
If anyone can shed any light on this measuring system I'd be very glad to hear from them