Genghis
is it an interesting measure of a pilot's experience to ask their "Brown number"
It would be one measure of a pilot's experience, but not a meaningful measure.
As
mikehallam said earlier:
Depends a bit on one's age and therefore access to some older types - as more recent a/c don't count.
There will be other factors.
eg Those of us who belonged to the Tiger Club in the 80s (and perhaps later) will have time in the Tiger Moth, various Jodels and Stampe SV.4. Those who didn't are less likely to have flown a Tiger Moth, and far less likely to have flown either the SV.4 or any model of Jodel - unless they have lived in France or Belgium.
Few people are likely to have flown the original Stampe et Vertongen SV.4 which Eric flew. Only about 30 were built before the company closed during WW2. Stampe et Renard renewed SV4 production after the war - as trainers for the Belgian Air Force - and others were built under licence in France by SNCAN. If I remember correctly, both the Tiger Club's Stampes were SNCAN aircraft.
eg Right place right time, friends who own and/or fly rare types.
I've been exceptionally lucky (for a PPL) in both those respects but it is not a meaningful indication of my experience as a pilot. I know people who've never flown anything other than s/e Pipers or Cessnas who have far greater experience as pilots than me.