Yes, very rare and in this case it seems to be unique to Dicksons.
Bell was a big fan of the 220 swift in later years when he returned to live in Scotland where he used the cartridge regularly for hunting red deer. He raved about the cartridge and its killing power.
Maybe that's why Dicksons decided to offer a modified MS to suit the 220 swift as that was about the same time as Bell was living back in Scotland. Interesting.

Matt.