Some time ago I contacted GSI (merkel importer) about importing double rifle in-the-white (for stocking & finishing here in the U.S.) We had several conversations over a month or two, but it didn't work out.

They indicated definitively that they were importing the actions for 20ga shotguns for resale to Rogue River (now Rigby)... Furthermore, they were unable to export "rifles" unfinished (apparently b/c of German proof-laws) but were able to export the shotguns unfinished - it was about a year or so back and I don't recall the exact reason for such. But the answer to your question (from GSI) is Yes, Rigby is/was using Merkel 20ga actions.

Recknagel (the German maker New England Custom Guns rifle sights) also makes a similar actions for sale to the trade... they even make the locks for a sidelock.