They did exist, but usually the lower (maybe 2/3rds) leg of the serpentine was waaaaaaaaaaaay longer, like a good foot or more below the stock. This was on purpose ...
for safety ... so that the heavier leg pulled the serpentine back, well away from the touch hole! Recall ... no pan cover on these fire locks! These also were more so done on the 'hand gonnes', before they became what one might call an arquebus.
Lodgewood has making a few MLocks that I can't find anything of historical precedence,
which I don't get ... as Steve K. there typically researches his arms really well, as well as provides the historical provenance.
Concept Pic Only
View attachment 277729
This one was made by Eric Kettenburg
View attachment 277728
Allegedly she is a replica of an early 15th Century firearm. This gun is a 77-cal smoothbore with an ash tiller. The picture of a
similar gun from 1411 is the oldest illustration of a serpentine lock firearm.
TIP - One can make one like above
for < $200 from The Rifle Shoppe 75-cal hand gonne kit #793; you'll just need to fab your own tiller (stock/pole/lance).