It's not a printing error. The cause is a missed envelope blank passing through the press, he inked image coming down on the embossing die and then the ink being deposited on the next blank through the press.
There is no catalog covering these, as far as I know. I collect these EFOs and when I bid for them I always have opposing bids. So, there's interest out there. Under-inked and over-inked examples are worth less than one like yours, which is inked like a post-office fresh "normal" example.
I would estimate a value range of $10 - $20 for yours.