ORIGINAL: HEAD0001
Where the bolt release is at was replaced by a small flat piece of steel that folds over to take the bolt out. The scope mounts on top of the flat piece of steel with two screw holes. Very similar to the Wincesterside mount scope mount. I assumed this was done by RR-it looks time period like??? It is held in place by a set screw. It may be a Remington, I am not sure. It is marked as a refurb in 1944 at Rock River Arsenal. I would have to check for the other markings. I will get it out one day a post a picture. The rifle is in my vault. I am pretty sure it is an 03A3-but I could be mistaken, I am not an expert on such things. Tom.
Ok I got ya now.....that's an aftermarket "drill free" scope mount. It fits in place of the cut off switch on the left side of the receiver. I don't even think it was available beck then.
The easy way to tell if it's an 03 or an 03A3 is the location of the rear sight. If it's in front of the receiver and has a graduated ladder it's an older 03. If it's located at the rear of the receiver (about where you describe the scope mount to be) it's an 03A3. Yours might have been removed to make room for the scope mount.
They are great rifles either way and it sounds like yours has not been permanently re-arranged and could be put back to mil spec if so wanted.