you can number the board, and the back of the print
you can sign a tag and number it on the back of the print
or a stamp
or a certificate ...

or you can just not differentiate between the proofs and the editions and anything else
anything can be considered an edition, a size difference, a different paper printed on, a different developer, toner

me, i just sell single editions, because .. i don't want to be tied down to make 25 or 10 or 50 of the same print,
i have better things to do ...

in the end, does it really matter ?

good luck with your sales!