It appears to be a montage of Belgian & German mechanic efforts. A Gesteck / parts kit was sourced from Liège, then Krupp tubes were added & finished in Suhl, Germany in 1931. Some of that British Chicken-Scratching obscures the original marks and I for one would say it is a Cardinal Sin to do so but the Birmingham(Guardians) proofmasters, along with the Worshipful Company of Gunmakers, are quite pompous and tend to suggest that only their marks are proper & apply.

Serbus,

Raimey
rse