Burgonet: Solothurn arsenal number?

[AHorsa]

Thank you Jasper, fantastic! It is exactly the same "3"! I don't know how far these old stamps vary - is it possible that this very stamp wwas used for my helmet as well or was it a common form of "3" and is just from a similar stamp set?
Can you tell the name of the book?


Here is one from Solothurn which - on the same position as mine - shows a "× 126":

https://european-armour.com/images/A-5.jpg

So at least we can say that the marked on this position and used this remarkable "3" as well as the "x"

Best regards
Andreas