Hi, On a 2001 E450 Bus, the sidewindows are, I believe, called "Top Double T-slider". So, the sliding part is at the top, the window appears symmetrical.
Question: Can I pull the windows, flip them upside down, so the slider area that opens will be on the bottom? This would enable my dogs to enjoy a breeze. If not, any replacement windows out there that would open low enough to enable my dogs to enjoy the breeze? I may buy this, but if my dogs can't have windows open, I'm not going to. For real...

 

My 95 E350 Bus has probably the same windows as yours.

They look like they can be flipped over as long as it is not the "escape" window. They are windows that have red handles so you can remove the big glass out and escape the bus if needed(also a source of bad rattles).

 

Thank you Eddiec, all but the escape window should be fine, it looked possible, maybe I'll buy and try, sell if I can't, haha.
Gotta spoil my dogs...