Mike
You may want to find the Museum Restoration Service monogram on "Sir Charles Ross and his Rifle" by R Phillips and J Knap. It appears that the Model R and E was apparently available in .303 Britishicon and .35 Winchester. Apparently, the Model R-1910 and E-10 were only available in .303 British and have the extended magazine. No mention is made of any rifle with a flush magazine based on the later action in anything other than in .280 Ross.