It is a 21 bore(@ time of proof) set of serialized tubes sourced from T. Kilby, more than likely in the white. Typical platform for makers on the Continent who would finish it as their sporting arm. Looks like the number >>20<< in the rhombus.

http://www.doublegunshop.com/forums/ubbt...true#Post231200

Cheers,

Raimey
rse