I have started compiling an overview of pre-WWII Couesnon Monopole Brass serial numbers, cross-referenced with the engraving and any "grenade" mark indicating the year. A notable observation so far is that the grenade mark with the year appears on Monopole instruments from 1921 onwards, but not before that; however, the engraving had already been expanded prior to that date. The years preceding 1921 are estimates based on average annual production figures, accounting for reduced output during the war years. It also appears that the numbering sequence was restarted around 1931.
I gathered the data using photos from the internet and instruments from my own collection. If anyone has any additions, I would be happy to hear from you.
I am particularly interested in the serial number ranges 18115–21470, 28349–30203, and those above 60499, as well as the point where numbering started over around 1931.

https://horn-u-copia.net/Docs/Monopole%20serials.pdf