I can assure you it was born as a shotgun. There are many different Victor Sarasqueta side-by-side shotguns and double rifles. Some models were of high quality and some not. Double rifles, although very similar, were made with different and stronger steel actions, and all I have seen have serial numbers 79XXX. This shotgun model, known in Spain as "the one with ducks", or something similar due to the engravings, was not of the "good" ones. I would like to see the markings and serial number engraved in the action and under the barrels, it was probably made in the 50`s or 60愀 and most certainly are not 79XXX.
The SxS Sarasqueta shotguns of that period were mostly made for 65mm shells (2 1/2") and proofed to 900 kg/cm2 or less. I would not be very comfortable shooting 375 H&H with a double made with one of this actions.

Best regards