Well that is confusing.
As you observed, the 20-70 in the Omega is definitely post-1924.
There are not however
lettre annales nor Smokeless Powder proof marks. Smokeless proof was obligatory post-1924.
Are the markings on the rib/barrels in English? It is not uncommon to see Belgian doubles intended for the U.S. market to not carry Smokeless Proof as there was an additional charge therefor.
'HS' or 'SH' could be the dealer? Spalding-Haywood Arms Co. was in Denver
The fluid steel barrels were by
Laurent Lochet-Habran who supplied tubes to most of the U.S. double gun makers, including Hunter Arms, Baker and Ithaca.
If the wall thicknesses are OK, I would not be worried about the absence of Smokeless Proof. The rough forged tubes WERE proved before the joining and finishing.
I would think that the gun would be shown in the 1926 catalog
https://www.cornellpubs.com/old-guns/item_desc.php?item_id=3618 
