I ended up buying this item. Having it in hands and checking the spacing as well as holding a reproduction rail made per exactly per the original specs (except for the fitting of the feet) I can confirm this was an original Ross Rifle M.1910 Sniper Rifle which had been fitted with a Canadiancontract Warner & Swasey Model of 1913 Musket Sight. It interestingly also got confirmed by the very uncommon and rather unique threading one would not had assumed they have used which also corresponds with the original drawings.
From now on it will take a long time until it will be fully restored since it had been stripped of many parts and it will take time to find all of the missing parts, as well as repair what Bubba had done to it (who isn't the previous owner, to make it clear). It however deserves being restored and I will do anything to bring it to former glory. Fortunately I already have the toughest part for it - the original Canadian contract scope.
Attached a single picture showing of how the holes in the rail perfectly align with the ones in the receiver. More when progress has happend, possibly then in an own thread.