Yes, the 1955 Target had a standard barrel length of 6-1/2 inches from 1955 until about 1979-1980, when S&W reduced the barrel length to 6 inches. By then, the 1955 Target had reached Model 25-2 in the model number scheme.
S&W never cataloged the 1955 Target in any barrel length other than 6 and 6-1/2 inches. Most guns seen with barrels shorter than that were after-market gunsmith jobs, but S&W would make up shorter barrel lengths for special orders occasionally. The ones I have seen that lettered were all 5 inchers, not 4 inchers.
I believe S&W made up a batch of 4 inch barrels for the 1955 Target that they would install on customers' guns. Doc44 has a 1950 Target that belonged to Remington Arms earlier that had one of these barrels installed later in it's life. I have a Model 25-2 that has one, but it letters as having shipped as a 6-1/2 inch gun. The markings on these 4 inchers is placed way differently than they would be if they had been just shortened, longer barrels.
I have long wondered why S&W didn't make the 1955 Targets with 4 inch barrels as catalog items. I believe that they probably had made a conscious marketing decision to push their "own" cartridges, the 3 Magnums, as holster guns for those wanting 4 inch, adjustable sight N frame holster guns, as the .357, .41 and .41 Magnum cartridges were linked to their developers, S&W, in the minds of most shooters, while the .45 ACP and .45 Colt were often linked mentally to S&W's competitor, Colt.