In the catalog of the perfins of the Netherlands and OR 3rd edition the perforations are displayed per perfin and per NVPH catalog number.
This has not been done for the airmail stamp NVPH no. 9, although this seal is known in two different types, namely 9A in comb toothing 14¼: 13¼ and 9B in line tooth 12½.

To improve the perfin catalogue we ask you to measure the airmail stamps no. 9 you possess and report the result to Jan Verhoeven by e-mail:
It is not necessary to work through your entire collection, because as far as known there are only seven different perfins in these air stamps: GWC, HC 1, S 2, VD 2, VH & C 1, VH & C 2 and VN 1.
The result of this inventory will be included in the next Perfinpost and shown on this website.