OK. "O.P." Stands for Original Post, meaning the first post in that thread. That is where he put all the 3D files, together, so they would be easy to find.
Look again as I think you are mixing up the filter (front of the bellows) with the hoses (back of the bellows). That looks great for the hose fitting at the back of the bellows. There is no "side pipe" needed for the filter fitting (I am pretty sure).
I didn't think it was a T angle either till I zoomed in on that other pic and saw that the bottom of the T attaches to metal strips attached to the chest piece
No, you are right that there are two fairly different versions of that front fitting. Judging by the blood, that photo is of the dead Joe figure they made and it is definitely much closer to your fitting.