Hard to say. More likely than not it's a 3520A as most of the 3500s are no longer in circulation at the B&M level as they've been discontinued. You'd have to open up the box and look at the model # on top of the case. If it says MD-16XDVD9A2 it's a 3520A, if it says MD-16xDVD9A then it's a 3500A.
I bought a Mad Dog Drive on President's sale a couple of weeks ago hoping to get a 3500A also but ended up with a 3520A. I'm not too upset as I already have a 3500A and this is going into another PC I'm building.
A note of caution buying online. I'd call before placing an order to check. One way you can ask the CSR is when the drives were made. If they were made earlier this year eg, Jan 05 then it's a 3520A. Also, before buying I'd check people's rating of the vendor at
Hope that helps a bit...Happy hunting...