The "Mo van Barr" glitch is a huge obstacle to people trying to get their pre-order bonus of the Deadly Assassin Outfit. One of the requirements to unlock this outfit is to find and capture/kill Mo van Barr. For some gamers Van Barr isn't showing up in the wanted posters, though you can keep collecting bounties and it might be possible you can get him later but won't be able to tell from the poster that it's him; not sure though, for others he escaped during the wanted mission never to re-appear for the rest of the game. While others downloaded the Deadly Assassin Outfit after killing Van Barr, therefore "putting the cart before the horse" and killing the target before he was eligible to be killed for the outfit. Another known issue is when you finally get him to appear, and you manage to catch him (or kill him) and you get no Deadly Assassin Outfit scrap.
Alternatively, you can start Marshal Johnson's first mission anytime without completing all of Bonnie MacFarlane's missions. All you need to do is complete her second mission "Obstacles in Our Path" to get the Marshal's first mission. Then, initiate "Political Realities in Armadillo" and kill Walton Lowe.
Sometimes it's possible to get the Scrap for the Deadly Assassin's outfit by simply playing a Bounty Mission in Armadillo after you've downloaded the outfit. If you've already done all the Bounty Missions, a wanted poster will show up in Armadillo with a question mark after you've downloaded the outfit. If you complete these bounties, one of them may end up being Mo van Barr, which will update your outfit scrap completions.
His name is accidentally misspelled in the menu map, showing "Moe van Barr."
His name suggests that he has Dutch ancestry.
Mo seems to be very mannerable, he calls Marston "sir" even while being pursued.
He is arguably the cleanest and least scruffy looking member of Walton's gang.
Mo is a unique character of the Walton's gang, on account of his (more) formal attire, as his hat, and dresswear don't show dirt or ripped clothes. He's also never seen in gang hideouts, leading some to suspect he's more of a figurehead in the group, that resides elsewhere.
Mo only appears as a bounty target, and cannot be found anywhere else in the game.
He is the only member of Walton's Gang who doesn't have a multiplayer counterpart.
Mo is probably a likeness of a legendary outlaw and assassin of the old west, ["Killin'" Jim Miller] . His similar appearance, well cut suit and top hat along with his manners and chivalry make him a mirror image of Miller.
Mo most likely takes over control of the Walton Gang after Walton is killed/captured, because he is the most educated member and already seems to serve as a figurehead.