They are shaped differently. The 7 has flanges with slots for bolting it down, is pretty square, and about the size of a Rubik's Cube.. The 6 has a stud sticking out of it and is more rectangular.
They use the same plug but wire up differently. Either will work as a bay timer, and there's a "flag" bracket that lets the LED6 mount on the same footprint as the LED7. They cost about the same new. If you plan to use them with Cryptopay, you need to make sure they're not LED3 which looks identical to the LED7.
If I’m not mistaken on the 3&7 the only difference that noticeable visually is on the wiring harness one wire is missing on the 3 that has a wire for 7 but I don’t remember exactly which one?
It's the 7 that has a blank spot on the plug, but if there's no plug in the timer that won't help. In fact the LED7 will work plugged into a LED3 harness.