Pretty much....Thanks to Dagga, you can see that the lower lug of the 3" USPS Speed Sixes have a subtly different curve at the front where it curves up to meet the muzzle. That's the easiest way to ID 'em vs. the standard 2.75" ones.
As far as I know (but who really knows with Ruger?...) the 3" USPS Speed Sixes were only available/produced in stainless steel.
Best, Jon