Picked this up too, think it could be a Punt Gun case, anybody any other ideas?
Totally unmarked, is certainly appears old, looks like the primer cup and cap have been replaced sometime in its life.
Head Diameter = 1.247"
Rim Thickness = 0.112"
Tube Dia under head = 1.152"
Tube Dia at mouth = 1.075" (Average)
Overall Length = 4.509" (As best as I can tell)
Edit; I did wonder if it had been turned as there are 3 dints on the rim, but have put them down to modifying the primer cup (could be wrong) ?