Most of the horns I have seen that are attached to the Possible Bag STRAP, have a simple hole punched in both sides of the strap, and then a thong, or belt run through the hole to the horn. I suppose, if you want to remove the horn from the bag, the easiest way would be to expand the loop at the neck of the horn, and pull the neck and stopper through the loop to free it. The same with the rear loop, or knot holding the base of the horn. If belts are used( I have only seen a couple like this in more than 25 years of looking) then unbuckle the straps, and remove the horn that way. If a simple knot is made around the strap using that punch hole, then until the knot and remove the horn that way.
I don't see any reason for removing a horn. I fill the horn from the top using a small funnel to guide the powder into the horn. I can do this with the horn attached to the bag strap, or with it as a separate unit, with a neck thong on the horn to carry the horn with a separate thong.