The credit card numbers are for accounts held with Visa, Mastercard, American Express, Solo, Switch, Delta and Maestro/Cirrus.
Within the address bars of the cached pages are URLs of companies, including UK retailers of laboratory supplies, sports and health goods, apparel, photo imaging and clothing...
"It looks like the site might have been a payment processing gateway that handled credit card transactions for a bunch of websites before it went belly-up," the worker speculated.
And there's this comment to cheer you up:
The worker tried to report the find immediately to Visa and Mastercard, which have the lion's share of card numbers, but said neither returned calls.