Love it! Thanks for showing it!
I believe you have a real old-timer there. Considering the condition, I would agree that it is most likely post-1850. However, there was an article in (I think)
The Museum of the Fur Trade Quarterly about roller buckles, and if I remember correctly they documented a bunch of these in various sizes being brought to the Rocky Mountain rendezvous, 1840 and earlier. Mountain man culture, if you can call it that, was an equestrian society, and I expect they spent a lot of time in saddle and harness repair. Spare buckles must have been in demand.
As for the harness rivets, we had a discussion about those last year, and
@Artificer documented their use well back into the 18th century, and possibly earlier (see
Copper Saddle Rivets). So the harness rivets and the roller buckle could have potentially been used earlier, but all things considered, I would agree with you on the dating of this pouch.
Based on the fineness of the stitching and the overall symmetry, I think this pouch was made by a professional saddler or cordwainer, although the strap may have been replaced or repaired in the field. Old newspapers have ads by gunsmiths and gun dealers, and these frequently list "shot pouches" or "bullet pouches" along with powder flasks and horns, caps, and so forth.
Thanks also for pointing out the percussion cap holder. I've been paying attention to those lately, and the inventiveness shown by our pioneer forbears in making these is remarkable. I've not seen one like that before. I don't think I've seen a flap closure like the one on that pouch, either. So simple, but it did the job.
Thanks for showing this interesting antique!
Notchy Bob