I know this is a hot topic, but I didn't see an answer to this particular question. Several years ago, I worked for a large company and I was issued an American Express card, to be used for business expenses. The card had my name and the company's name on it. The statement came to my house, and I was responsible for paying it, and then getting reimbursed. I don't recall what kind of card it was exactly, but I think it was their basic green charge card. When I left the company, I turned the card in, and never heard anything else from Amex.
The account does not show up on any of my credit reports, though I have no idea if it showed up while I had the card. I assume that means that I won't be able to have any Amex cards I might get in the future backdated to the date I got that corporate card, correct? Though it's always worth a shot to try anyway.
If you worked there several years ago (as in less than 10 years), it should still show as a closed acct on your CR if it was opened under you. Since it does not show, it was probably not opened under you but through the company. If you still have an old statement you could try to get it backdated, but if you do not you probably won't have any luck as your past company held the real financial responsibility on the card (as in if you didn't pay they would inevitably go after your old company).
In all likelihood, you were an AU, or in Amex lingo, an additional cardholder. this is different than a member. What we call backdating on the forum is really just reflecting one's member since date. AUs, or addtl cardholders, are not considered members. So any Amex you get would not reflect your AU date.