The incident that I witnessed was in Cherokee, Iowa at their annual Fall rendezvous. The dude that forgot to stopper his horn was a past president of their club and had a lot more gray in his beard than I did......at the time. 
I just received my latest copy of MUZZLELOADER MAGAZINE which included a spread on the horn-work of a member of their guild.....BEAUTIFUL workmanship by a true artist. Not one of them had a tethered plug....nor will you find any (or very few) examples of the originals that had them. Check out Madison Grant's books on the Kentucky Rifle Hunting Pouch and its accouterments.
Losing a horn plug ain't that big of a deal.....they grow on trees in my neck o' the woods. 
Seems we have a difference of opinion which in and of itself is a healthy thing, unless or until it becomes arguable, then it becomes less about the the thing and more about ones ego, now I've read Madison Grants books on horns and on hunting bags when I was making powder horns and hunting bags and I honestly don't remember seeing or not seeing tethered plugs, mostly because my choice to tether mine is based more on practicality than historical correctness.
Now my old friend, horn spout stoppers may very well grow on trees in your neck of the woods, but mine are usually hand carved and matched to the horn, loosing one is a real loss of time and creativity, as is the possible loss of the powder content of the horn if one is not immediately aware that it's been lost, oh I'm sure that there is a historically correct answer to whether plugs were ever tethered back in the day, and it may matter greatly to Stitch Counters everywhere, but I happen to be the last person in the world who cares what history thinks of tethering a horn plug, and certainly not enough to argue the point seriously with an old friend.
In my reply post I credited you for your point well taken and then rendered my opinion along with my reasons for bearing that opinion, that should have if not agreed with then at least respected as I did with yours, so my friend (and I am speaking to a friend), if you feel the need to argue the point further you'll have to find someone else who feels it's important enough to argue about, I'm not your guy.