If the enlongated shield w/ a crowned "S" or it looks to be a crowned "S" in a lozenge, I think that to be the mark of the Schilling forge. If you get a good pic of it, I'd like to see the image. That's the area where you generally find other initials.

Yes, I don't see any indication that the chambers are any longer than 65mm and generally the forcing cones were long. So it you find an example that is 70mm and shorter forcing cone, then it has ben altered. The left tube is choked and the right tube has/had less than 0.2mm of constriction. It is a roundtop version with the caveman tradmark stamped atop the top rib just ahead of the Greener crossbolt.

Kind Regards,

Raimey
rse