"Wyatt said a special branding iron was there that was specially made to make US into D8 in one burn. Most rustlers knew to use a running iron because it would do a cleaner job and making a complete branding iron for only 6 mules would be a waste of time and guarantee a blurred brand."
I would like to know your source for stating that a running iron would do a cleaner job, particularly if one wanted the brands to be as identical as possible. (Source Joyce's many years living on a cattle ranch with a cowboy and living that life.)
According to some sources, even having a running iron in one's possession could be reason for a quick appointment with a tree limb and a rope (google "running iron"). (I believe that is correct but I believe the total loss of ranchers and outlaws south of Tombstone over rustling apparatus was zero?) Correction if you have one?) In terms of whether making a DS branding iron would be worth it, based on the following referenced article, you be the judge. On May 18, 1867 the Arizona Miner reported the theft of five “splendid mules” by Indians from Toll Gate Station, valued at $1,200 ($25,056 in today’s dollars). That was 5 mules in 1867, when there were fewer people and mules in Arizona, so the value may have gone down, but I imagine 6 government mules in 1881 would be worth a pretty penny. If the price of mules, especially mules that had been certified for government service by an Army purchasing agent, had dropped by 50% by 1881, that would still be in excess of 13-14 thousand dollars in today's money for six mules. Cut that in half again, would it be "a waste of time" (your words) to make a special branding iron for 5 to 6 thousand grand of free money? (Lots of figures there. I asked what a mule would cost now and Google said 1000 to 8000 with a $3000 average. Divide by 30 and you have $100$ cost back then. But the point about cost may be something to consider? It would stil leave you with a blurred brand though which might not go over well with the buyer?)
Tom, you say, "Also US mules would have a second brand on them. The US would be on the right side of the mule near his front shoulder. Another brand would be on the left side near his rear shoulder naming the military unit that owned him.
"This being true how could Wyatt think changing just one brand would hide the true ownership of the mule?"
(That is the point. Wyatt didn't know and thought the D8 would do the whole job. When Wyatt was stealing horses when he was younger, they were privately owned and probably had only one brand.)
I have to thank Joyce for reminding me about the "card" placed by Lt. Hurst in the EPITAPH that helped me to answer this one. In describing the mules, the only brand he refers to is the "U.S.". He makes no mention of another more complex brand that would have been even harder to alter than the simple "U.S.". To pose your question in a slightly different manner: This being true how could Lt. Hurst think changing just one brand would hide the true ownership of the mule? (Hurst didn't comment on the second brand one way or the other. Perhaps he was hoping the rustlers were as uninformed as Wyatt and it might help him identify the mules later?) The mules obviously did not have the second brand on them) or Hurst would have mentioned it. (why?) Also, why would the Cowboys steal six double branded mules in the first place? (Perhaps they didn't know any more than Wyatt or intended to blur the brand and claim it was too small to do right?)?) If so, how would they intend to cover up the one identifying the unit? (If they knew they would need a running iron or another specially made iron to blur the brands and maybe we are not to know?