Other than the cosmetic issues of the sheet steel lower metal and barrel bands,
the sights are the difference.
The '03 has a standard-for-the-era "middle sight" which, a bit different than later
Mausers, pops straight up, the slider running up and down. There is a peep hole in
the slider, and a notch on the top, plus the notch which is 90 deg to that, for use
when the sight is folded down (battle sight). The peep works well, but is a bit odd,
being so far forward.
The 03A3 has a ear-protected peep at the rear action ring, the 'right place" IMO.
It is readily click adjustable for windage, very nice. It has no zeroing capability for
elevation, you file or replace front sights to get elevation zero. The peep portion
slides on a short (~1") ramp with a click-in-position location for each 100 yds, from
100 out to 800, IIRC. Nice, tough little sight, excellent sight picture, fast and positive
for ranging, once zeroed for std M2 ball ammo.
IMO, for a target shooter, they are about roughly equal, although with an add-on
external micrometer adjuster, the '03 rear can be finely adjusted for target work
in elevation, less precisely controlled in windage, although readily adjustable. '03A3
has no fine elevation adj, but great windage adj. Better peep view and quicker, too.
To hunt, '03A3 is far better. For a battle rifle, the '03A3 is a definite improvement.
Bill