CotW says 10.3x60R Swiss is the Continental version of the British (mostly Scottish) .450/.400 2 3/8" BPE.
Mostly seen in Martini single shots, no mention of a repeater.
Still available in Switzerland, required for hunting in one canton there.
I don't know how to get one in the USA.
CotW says the RWS smokeless load is comparable to .405 Win.
Which IS available here.