I can't be sure from the picture, but it DOES look like you have a half-rring-shaped square-edged cut in the correct vicinity. Usually, the flange cut ends up a bit wider than the profile of the OD of the receiver ring, though. This one is indeterminate, to me.
I checked some pictures in Brophy's book, and they were actually not all that clear, either, that I could say one way or the other from them..
However, I COULD see that the area where the 'ring cut' appears to be does have a more sharply defined profile in that area than on the older 03-only stocks shown in Brophy. From what I can tell, the 03 stocks have a somewhat more rounded profile that runs into that ring cut area. .
You can probably more easily determine this yourself.
Does the rear 'flange cut' on the handguard line up with this 'groove' when the handguard is being held on the rifle in the proper position, as in the two 'assembled stock'' pictures you posted?
If the flange cut on the handguard lines up with the ring cut on the stock, it should be a late (useable on both the 03 and the 03A3) stock.