So you use it to extend dives - which isn’t what a pony is for - so you won’t call it a pony…and it’s not a stage bottle either…so you make up a silly name for it that no one else uses. That way when you’re challenged on how you use it you can claim some unique, totally fine justification because “hey, it’s a “stony” tank” which is really just a meaningless term.What I have found is that when a debate does ensue over using a second bottle to extend a dive, it cannot be referred to as a pony bottle, hence the name "stony" bottle which is a combination of stage + pony. Yes I know it's not really a stage bottle when used in that fashion either but the pneumonic indicates that the bottle has some sort of mixed use purpose.