David sent me the photo of this little thing in better resolution. There is the name on ribbon F.O.Matska.
I guess it was very serious reason Russian maker put his name on Tzar' gun previously known as Lebeau gun.
I need additional information about proof marks and signs.
But from what I see now this gun been executed by Matska or at least he made very seriuos custom tuning to this gun.

Geno.