Roy Jinks' book THE HISTORY OF SMITH & WESSON states that the .45 Target Model of 1950 was made with no model number, beginning in February, 1950, and then Model 26 (model numbers began in 1957), 26-1 and 26-2, the last two variations being pretty unusual "and offer the collector a real challenge." The gun was dropped from the catalog in 1961, having been displaced by the .45 Target Model of 1955, with it's heavier barrel, which gun was assigned as Model 25 in 1957.
Jinks wrote that there were 2,768 .45 1950 Targets made before being discontinued.
So, yes, it is very probably authentic. And, probably one of the very last 1950 Target .45's made.
The Model 26-1 that was mentioned by Tom R was a different gun altogether. SCS&W III calls it the 1950 Reintroduction: Georgia State Patrol Commemorative. Manufactured in 1988-1989, it was a 5 inch gun chambered in .45 Colt. 800 were made, all but 40 having the gold-colored filled markings for the commemorative, smooth Magna stocks and a lanyard swivel in the butt. These were marked Model 26-1 but were built using the mechanical changes and updates of other N frames of that era, such as a non-pinned barrel. Had S&W followed their normal numbering scheme, this Commemorative .45 should have been model marked the 26-3, to my way of thinking.
And, it is REALLY worth a factory letter to confirm this. If you ever want to sell it, the letter will pay for itself numerous times over.