I've had a TSU3500 for 9 years - happy owner.
I'm about to get a used TSU7500 from an eBay seller. Seller cannot determine whether it was modified with ProntoPro Edit. I sent him screen pics from my TSU3500 (unmodified with ProntoPro Edit) for comparison.
My Newbie question: If TSU7500 was modified with ProntoPro Edit, how do I undo those modifications without access to ProntoPro Edit - I have a Mac OS10.10 laptop, PP E only works with Windows XP etc. - not Mac ay all. I know there's an app Tonto, but doesn't work with NG Prontos from what I've read here.
The reset button in the battery compartment from what the manual states does not clear this.
Any help appreciated.

You will not be able to upload/download configuration files to that remote without ProntoProEdit Next Generation (PPENG). I think some have been successful here with Parallels on the Mac.

Tonto only works with the first Gen Pronto TSU1000/2000/6000 and EU derivates of those remotes. The Marantz Equivalents use TouchScreenSetup (RC5000/5000i/5200/9200).

You can put a 3500 config on the 7500 but it will be migrated because the Black And White 3500 uses a palette-based transparency scheme whereas the 7500 uses Portable Network Graphics (PNG) files that have a transparency layer. As such, any 3500 config you load, you will likely have to change images, expecially if your buttons have rounded corners, etc...