I'm late to this one, but I carry my 4" Service Six every day in a Bianchi holster marked for a " 4" S&W K "
Also have quite a few duty holsters, the ones marked for S&W 10-64 fit my Service Sixes perfectly, and the S&W 19-66 holster fit the Security Six.
For future reference S&W L-frame holsters fit the GP100 and N-Frame holsters fit the Redhawk, but the Redhawk is wider and it's kinda tight.