Sorry Nate, this does NOT seem safe!
Might be but would need PROOF!
I did just look through the Milton Catalog and see nothing listed above 250psi. They could have an unlisted fitting but unless it is STAMPED for +5,000psi don't go there.
Then, hooking your buddy bottle to rifle. Is okay UNLESS the reg. ( common 2,900psi say on the Ninja/Crosman ) is set above the working pressure. A 2,900psi regulated bottle on a 2,000psi Maximus, dont do it.
You are close but you are also stating these are instructional video's and perhaps others should try this also. Be very careful what you say, you may know whats what but people will see something on the Internet, think they know it all/enough then rush ahead, get hurt and blame you.
? things eh? Is there some reason more than 1 person has instantly spotted that fitting? "Could" they be right? Is every metal part your used stamped/marked for the rated working psi? At least came with documents stating the max & working psi?
John