David Tulk of Madelena suggests they are introduction sequence numbers.
“Mold numbers they are not. One number appears on several different molds. Palette numbers they are not, nor artist numbers. The apparent progression in style however - jerky, but a progression nonetheless - doubtless mirroring developments in the tastes and fashions of the period - might support the notion that the sequence of numbers is chronological. Starting out with traditional style figures (1,2,9), it would seem the range was periodically enlivened with the introduction of new subjects (20 onwards) to maintain appeal. A more homely style becomes apparent in the choice of some subjects introduced later (50, 89, 90, 133, 134, 153, 154). Towards the end of the sequence are some that could be described as true to life naturalistic figures (150, 153, 154, 164, 166). My suggestion is that they are introduction sequence numbers. Most likely used when production first started it is anyone’s guess as to why they were abandoned in later years.”