Wait a minute - looked at photos closer - the barrel and receiver has the same Spanish serial number. This gun cannot be in 6.5 Swede unless it has been sleeved. It is most likely 7mm Mauser, or possibly a later 308 conversion. If it has been sleeved, then it is totally dubious.
DO NOT FIRE THIS RIFLE - Take it to a gunsmith and have a chamber cast. Better yet, dispose of it.
1) It is a Spanish Receiver and barrel.
2) Gas relief hole has no relivence to 7mm or 308. This was done much earlier than the 308 conversions.
3) The original 7mm chambering is a bigger diameter bullet than 6.5 Swede. DO NOT FIRE 6.5 SWEDE AMMO IN THIS RIFLE!!!!!!!
4) Again, I emphasis, all consideration of collectable aside, you have bought somebody's home workshop sporter. I seriously doubt the 6.5 ammo was fired in it. The Garage sale folks were just trying to make a buck, and could care less if you blow your head off.
OK, let me try to explain again - 6.5 is slightly smaller in diameter than 7 mm. If you are firing a 6.5 diameter bullet through a 7mm bore, it does not contact the grooves and lands and is "free floating" down the barrel, in which case you have very little accuracy. Now, to sleeve an original 7mm barrel to 6.5 - I think that is a very hard thing to do, and will likely not be a very competent job that will hold up.
I don't know what these 3 different people that looked at it used to judge that it is 6.5mm. Frankly, I would not have any faith in some hybrid monster unless I knew the person that assembled it, and knew of the quality of work they did.