There is no difference at all.

The outer diameter of the rear base sleeve is in both rifles (10,75 and 9,3x62) 25,04 mm(ca 0.986).
Wall thickness of that sleeve round about 1mm .
So the corresponding barrel diameter on that portion is 23 mm (0.905 ).

The barrel diameter starts with 21,8 mm (0.859 ).

Taper differs (different muzzle diameter) 10,75 round about 17 mm.
9,3x62 : straight on the muzzle 14,5 mm and where sleeve of the frontsight ends(28mm back from the muzzle) 15 mm .

So if you want to copy the Osa rifle you have to choose a barrel shank diameter of 29 mm , length 36,8/37 mm.

Your "second" step ends 144,4 mm apart from the front receiver ring.

Set back the rear sight sleeve 4mm from that step ( and be assured that all backsight sleeves of the A Type have a nominal length of 80 mm = starting from the Kurz Type A Mausers up to the .404=).

Set it flush with the step and you have the other version not so liked by you but also very common.

Measurements taken from a Type A 9,3x62 (1932).

Have in mind that measurements from real pieces may differ slightly because of the minor differences in the finishing process , that was a handjob in those days.