What's the password to the System keychain?

Some background:

I first got a Macbook Pro, 1, from my work. It was connected to the company AD. Since I had admin rights on this machine and didn't like having different AD policies forced upon me I reinstalled it to get a plain Mac OS X, free from any AD connections. 1 was then involved in an accident including water and broke. I then got a replacement Macbook, 2, which I reinstalled to get out of the AD like I did with 1. I then and transferred all data using Migration Assistant (the harddrive of 1 was not broken, however, the file system needed extensive repair).

So far, I didn't have any problems with my System Keychain. However, I then got rid of 2, after doing a Time Machine backup of it, and got a third Macbook, 3, which I wanted to do a clean install on.

Now when I try to read some wifi-passwords stored in the System keychain for 1 and 2 (by browsing to it on 1's old hard drive/the Time Machine backup of 2) on 3 I get an error message that the password I enter is incorrect. I am as sure as you could possibly be that I know the password to the account I had on 1 and 2 (I use the same password on 3) but obviously this is not the correct password to unlock the System keychain.

If I open my user keychain from 2 (from the Time Machine backup) I can unlock it using the password I used on my account on 2 (and 1 and 3). However, if I open the user keychain from 1's old harddrive I can't unlock it using the account password I used on 1. 1's water accident led to a lot of write errors on the harddrive and a lot of orphaned files but since all 1's keychains have worked on 2 I don't think the keychain files were affected.

So my question is something like: how does the OS unlock the System keychain after I login using my account password? Where does the OS find the password for the System keychain? In my user keychain? Apparantly, using the Migrations Assistant, iunlocking System is a non issue, while when I copy the keychain files manually something that is required is not transfered.

A final note: when I got 1 it probably came with some default password. It was 3 years ago so I don't remember. Can that be relevant here? Since I did a clean install (IIRC) I would be surprised but I guess there are different ways to reinstall a computer and I don't remember exactly how I did.