The shoulder diameter of .448" is quite a bit larger than 35 Winchester, which is .412". The cartridge is darned straight sided. A chamber cast would be helpful.
Those case body dimensions correspond closely to my measurements of a fired .303 Epps case. The neck dia. will of course be different. The short steep shoulder appears to correspond as well. IMO that's a .35/303 Epps cartridge.
Provided you are not set on the 35 cal, your best and least expensive option for getting it in shooting condition might be to re-barrel it back to 303 British. Decent Mk II barrels do show up. The rifle has a left hand 3 TPI thread that was designed for easy barrel replacement. I assume, but am not certain, head spacing would remain unchanged. Since the present barrel could be easily reinstalled, any collector value would not be negatively impacted.
Getting the cerrosafe into the rifling should not lock it in place. I routinely include a bit of rifling when doing a chamber cast. Did you try to remove it within 1/2 hour after casting? That is pretty important since it initially shrinks and then starts to swell back to original dimension shortly thereafter.
I don't have a diagram but to my knowledge the variants on the Epps case were all necked up from .303 to the appropriate dia with no other changes.Well I screwed up my chamber casting. The cerrosafe musta got into the rifling and it wont come out now. I'll heat it up tomorrow and get it out.
I have a 303 EPPS in a 1910 Ross done by Elwwood Grandson, just South of Gravenhurst on the North Bound side.
I agree, it does look a lot like it. Are there any pic's/diagram's of the 35-303?



























