Numbers or letters on used kegs

[aerod1]

I see used kegs for sale and they will have a number like #1 or #4 or #5 or E listed . What does that mean ? Thanks .

 

[day_trippr]

I'm guessing those are "grade" references, with a #1 being pretty near pristine while a #5 could look like a beater and may be missing a handle or worse...

Cheers!