Hi,
It sounds like you might have a #6 clone and, while that is normally considered a fore plane, can be used for jointing especially in shorter stock. I honestly couldn’t see the holes you mentioned in the photo but it was, as you suggest, a common practice to place a fence on planes being used for jointing. In order to overcome the step between the plane side and the blade, it was common practice to put a rabbet on the fence area that went against the plane cheek to give it the necessary offset. While the fence was more often simply clamped onto the plane, I see no reason why the previous owner couldn’t have modified it using tapped screw holes just as someone put a hole in the sole, presumably so the plane could be hung up on a hook.
Have fun