Eibar vintage single shot identification?

[Daveydave]

Good evening all. I’m now the proud owner of this vintage external hammer folding shotgun and would appreciate some help in further identifying and dating it.
dainty little thing- weighs 4lb.
It has Eibar proof marks (found online) and is stamped:
“Made in Spain”. (In English)
And ‘guaranteed for smokeless powder’ (in Spanish.)
850kg. (Proofed to?)
316. (?)
HG. (The gunsmith?)
And has a small stamp that says 12/50 or 12X50. (I assume this is mm? There is no reference to calibre otherwise.
Any thoughts? Thanks!
Dave

 

[tiriaq]

Would 12x50 be 2" .410?

 

[Daveydave]

Would 12x50 be 2" .410

That’s what I figured, but wasn’t sure there wasn’t some similar some archaic European calibre. The proof marks seem to date it to the 1920’s, and the smokeless powder reference suggests/supports early 20th century but I’m not 100% on any of that.

 

[Horilka]

X in a shield with knight head is admission to proof house introduced in 1931. Year could be coded with letter with * above, I don't see such marking on your pictures.

 

[Upland28]

Would 12x50 be 2" .410?

Probably. In Europe way back, .410 was also called 12mm, maybe after the size of the brass.

 

[burns k31]

Neat that it has a side lever.
That barrel looks long.
30 inch?