looking for a good shoulder holster for the above gun, preferably leather and on that can be secured to a belt.  Any suggestions ?

Note - I'm not looking to conceal this weapon, just something to carry it securely over winter clothing in while hunting in the mountains of Central Montana for protection.

Wearing this over a winter jacket isn't the best way to protect the gun against the elements, IMO.

A belt holster or even a fanny pack would be a much better choice, IMO.

But if you're hellbent on a shoulder holster, try the usual suspects: Galco, Kramer, Bianchi, etc.

Shoulder holsters seem to be a very subjective thing.  You either love them or hate them.  I go towards the hate them side.  Owned maybe a half a dozen over the years.  The only one I had any use for was the old Bianchi upside down rig for a Colt Detective Special and that rig won't do you any good for your intended purpose.  

Like Bob said I'm not sure a shoulder rig is the way to go.  If you're dead set on it, I'd maybe look at one of the over the shoulder rigs Sportsman Guide sells.  You might even be able get the Smith in one of the old M-3 military rigs.  Most of the Galco, Kramer etal all stuff will be designed for concealed carry.  It may not have enough adjustment in the straps to fit comfortably over winter clothes.  

I ordered This El Paso Saddelry "1942" holster (shown here w/M19 4")

for my M29 6"