Seems reasonable. Also, if it was, as Paul suggested, "the front half of an AEC Mk II turret", give it inferior turret front armor?
I honestly don't know that it was. I don't remember a photo with a frontal aspect. From that rear angle the turret front looks
vaguely like the AEC Mk II turret front and about the same size. Given the few conversions done by the Partisans, it could be anything, You would think that with the gun coming from an AEC Mk II then the internal trunnions, elevating gear and mounting frame would come from the AEC. Why not cut down the S-35 turret to its turret ring and mount the AEC turret on that stub, unless the rear half of the turret was damaged?
While it could be the front half of the AEC turret, it is more likely an extension made from scrap armour from the likes of SdKfz 222 or 251 or even unarmoured but high tensile steel. I was never a welder but I gather that the difficulty of welding plate goes up quite dramatically with the thickness of the plate. So my gut feeling would be an armour of about 20-40mm (2-4 AF), but I'm whistling completely in the dark here.
Maybe treat the Somua like the KV-2, change TCA pays non-turret penalties. ROF 0, No IF, No B(F)F, BU firer, B10.
While that turret suggests some of the problems in common with a KV-2, we're talking a much, much smaller turret and gun, almost a magnitude less metal
NT TCA penalty: Seems reasonable, but otherwise a 1MT.
ROF 0: Agreed.
No IF: Possibly, but I'd be inclined to allow IF due to the much, much smaller 1 piece 57mm shell as opposed to the enormous 2 piece 152mm.
No B(F)F: I'd be inclined to No BFF but allow BF. Aiming could be slower so moving fire would be impractical, but short halt should be doable.
BU firer: It's a 1MT, so BU fire.
B10: That's more than a bit overboard. If the dreadful Italian and Japanese MG only get hit with a B11, then this should get B11. I just can't see something getting out of a workshop and test process so bad it would earn a B10. The only ordnance I can remember with B10 are the Finnish ex-Russian field pieces from the 1880s that had no recoil mechanisms. ASL's B# are simply overly crude steps.
All the above are purest speculation. It could be that the turret was imbalanced but not enough to seriously impair traverse, even fine traverse. The source of the extension is unknown, so could be plate from an AEC, a Stuart, a German AC (~20mm), Pz III/IV, StuG III (30mm) or Panther side armour (40mm). So anything from an AF of 2-4.