These cels were described as 'promotional cels'
and it was mentioned how many were made.

The separate background does not mean it's
'multilayered.'  That term usually refers to
multiple sheets of acetate on top of each other. 
You can mention it comes with separate background
(printed, not painted, if I recall correctly).

If you can, you might want to list measurements as
well, even if it's just the matting.

Keys