JAF,
Have never had the Gov't Model, but did have a bull barrel 5.5" barrel Mk2 in stainless.
Oddly enough there was a leather Bianchi or Galco holster made for a revolver (perhaps a single-action revolver?) that fit that fit that gun nearly perfectly. The snap-over strap went snugly over the curve in the backstrap of the gun and secured it nicely.
Sadly, I no longer have gun/holster, so I can't tell you which model the holster was. But if you have a dealer who will let you take the holsters out of their packaging and test the fit using one of his in-stock guns, you might fool around a bit and find a holster that fits your Ruger.
Mark