Well just for giggles, and to give you something to think about.
A discrepancy I've heard, was it 13 teams or 13 wagons? 13 wagons would take one heck of a plan to eliminate all those troops and make off with the wagons. Like said, that's a lot of bodies to just up and disappear/ be buried without some word of it leaking out.
Now then 13 teams may be more likely, if the story is true, a heavy loaded wagon may need 13 teams of horses/mules to pull it. Anyone remember the 20 mule trains that hauled Borax from the mines? "Death Valley Days"
A single wagon would be a lot easier to ride off with, and return to a drop off point.
Also 13 wagons vs 13 teams, you are talking at least 13 bad guys if it's 13 wagons. These guys were not Navy Seals or Army Special Forces. Be a bit of a job to silence 13 guards at one instant. One boo boo, and someone is going to shout for help, or fire a weapon. Even if it was only one guard for 13 wagons, which seems kinda silly, there's something else.
13 wagons, means a minimum of 26 animals to pull em. At night the animals would be unhitched and corralled or hobbled. So you would have to have 13 bad guys take out one guard, or a whole bunch of them, then round up and hitch up 26 animals to the wagons. Then ride off into the night. Umm, 26 animals and 13 wagons are going to make some noise. You do not whisper to said animals to get em going, you slap em with the reins. More noise made, and not waking up anyone?
And this was a secret mission, yet someone knew of it, set up a group to overtake it, and disappear with it. That takes some real planning. The initial attack, the gathering of the animals, hauling the wagons out of town, then either dumping in a pre-planned spot or spots, hiding the spot(s), then 13 drivers to take the wagons back (really doubt they hitched all the wagons together for one guy to haul back), then say 13 horses to carry the drivers off. Heck even if they walked, 13 folks leave tracks. 13 horses would leave some serious sets of tracks.
Okay so they pulled an Oceans 13, and did all that, and no one notices till the next morning. 13 wagons gone into the night, dead body(s) laying all around.
A horse drawn wagon, heavily loaded, isn't going to make a long distance in even we'll say 12 hours head start. You have to subtract the time to unload and the time to get the wagons back to where they were found. So the drop off would have to be fairly close by if it was 13 wagons.
So you say they split up the boxes, okay, 13 wagons meet up with how many other wagons to divvy up the load. Even if it's 13 minimum, you're talking about 26 wagons, 52 animals, and how many people? That much stuff showing up in one place is going to tear up some ground. And if they were going to scatter, more then likely it would be close to a crossroads so they could head off in different directions. Lot less likely for someone to take notice of 2 or 3 wagons going down a road, vs 13(+) in a convoy.
And not one coin has ever shown up? Okay supposedly what 3 or 4 that there is no real documented proof of. So's 13(?) guys all killed except one who heads off to England. And not one person ever told anyone about this going down. Not a drunken brag in a saloon, or a whisper to a lady friend? No one skimmed a little of the goodies and pocketed it before burying the rest? Really?
Course I'm just a scarecrow hanging on a post who's got straw for brains.