9mm
They made two. The 547 is a blued fixed sight k-frame that uses some fancy berillium springs and two firing pins. The 940 is a centennial stainless that uses moonclips.
I own both, and paid quite a premium for these no longer sold models. The main issue is that 9mm is a rimless cartridge with a tapered sidewall. To keep it in place requires some engineering most revolvers dont require.
I love both my 940 and 547. They are rare though, so if you see one -- buy it.