6 1/2" barrels on this model were a standard length until about 1909. Then the extra 1/2" was dropped in favor of a 6" barrel. It appears that yours is the nominal 6" barrel. Because of fitment issues related to the barrel/cylinder gap, many of the barrels were not exactly 6" or 4" or whatever.
A good, more modern, example is the nominal 2" barrels on such guns as the Chiefs Special. The nominal 2" barrel almost always measures 1 7/8".
