Hmmm, there seems to be some promo card numbering weirdness going on.
I looked at the list of promos on dtdb, the numbers on dtdb are the same as the numbers on the card images for each OP card. Now, here is where the weird part starts, on my promos, the numbers are different. For example, Pistol Whip is OP1 on my copies, Pony Express is OP2, Stakes Just Rose is OP3, Circle M Ranch is OP4. On dtdb Pony is OP1, Circle is OP2, Carter's is OP3 and Stakes is OP4.
So it seems that the numbering on my cards (the real cards?) are in order of release with the common card being the lower number and the rare card being the latter number. On dtdb card images the order is instead Diamonds A-K, Clubs A-K, Spades A-K (? there is no heart "yet") but this does not hold up to the latest OP kit and the Sheriff's Promos, the Sheriff's promos I own have the same numbers as on dtdb and the latest kit I cant say since I do not own said cards yet.
There also is a "hole" that is too big for the cards I'm missing, so I believe that the next OP kit (after Unprepared and B&B Attorneys OP kit) will have two cards (like all but 1 previous kit) and that they are numbered before the Sheriff's cards.