Absolutely, regarding the double or single legal length doubles or singles, but I was wondering about the older vintage doubles, not the damascus variety as those have well discussed elsewhere. A friend has a couple of 20/12 adapters he uses in his grandads old shotgun.
These shells produce modern SAAMI-standard pressures. They are NOT low-pressure loads. But if an old shotgun is safe with other modern ammo, it will be safe with these.