Nifty marks JC. Is that a "Crown" over "GR" in an oval noting that Gastinne - Renette made the tubes. All firearms merchants were master gunsmiths. All master gunsmiths either made the transition to a firearms merchant or handed the reins to a successor. In keeping things in context, the example in question was sourced and G-R was more of a firearms merchant in the 1920s than a master gunsmith shop and it was pretty much due to economics. Many of the odd names and locations found on the top rib were mainly firearms merchants with a very small repair shop, which more than likely outsourced major repairs. Vienna sourced Suhl & Praha, who in turn sourced Wiepert and later Belgium. Paris was no different and I can't remember when the proofhouse closed but that had to be a result of the lack of examples passing thru the proofhouse after reciprocity was in place.
Kind Regards,
Raimey
rse