I purchased one on ebay advertised as "new un-opened" 512 mb 40x Dane-elec CF.

I was emailed that he had sandisk instead of the Dane0elec but both use the same Toshiba chips. I told him fine as long as it was the same speed also. I got it today but it came in "bulk packaging", in other words no packaging but the plastic "jewel case" they usually come with inside of the retail blister pack.

Alls it says is Sandisk 512mb compact flash. So how do I know if it's a 40x card or not?

What manufactorers stamp is this AX0403UFO. That is what is etched on the side of the card.

TIA

From what I can tell from their sight, they don't give their cards a X grade. I have talked to people about this 40x and 4x and so on and there is thought that it is just smoke for selling product, but I do not say for certain. As for preformance, I use Lexar professional 40x 512 as well have the 4x 256 Lexar and the 256 SanDisk and can't tell much between them. If this helps, difference except in price that is.

I don't know for sure, but I reckon the only time you'd really, really care about the write speed of the card is if you were shooting in 'burst' mode on a big pixel camera.

Some here have said there's a bit of difference between cards in how quickly they upload to your computer, as well. Today I uploaded a full 1gb 40x Lexar CF card via USB2... took between 10 and 15 minutes, I reckon.