I have a Gibbs 256 Magnum sporter rifle on a 1903 (Greek) M/S bbld action.
I think the left wall of the action is marked Steyr 1907.
The Swedish Mauser case base dia seems too large for this one also.
I'd read where the 256 Gibbs Magnum was a shortened 6.5 Swede,,but mine is a x57 case dia at the base. I haven't gotten back to attempting to make cases for it yet. Too many other projects.

A nice rifle, shows use but an excellent bore. An Aldis Bros scope in QD mounts added sometime after it left Gibbs I think as the View proof on the recv'r ring is under the slight over hang of the front scope mt base.
$400 was the price Laying on a gunshow table last spring and no one had wanted her by late in the day on a Saturday. Once in a while you find a good one.