Hello there!
I found a lot of threads on this, but none with the actual answer.
I'm at the stage on the DS-160 where I have to fill my prior visa number. I have a BCC (Border Crossing Card), and on the form it says there's a 12 digits number on the first line of the MRZ (Machine Reading Zone) on the back of the card.
The problem is that there's no 12 digit code. There are 12 alphanumeric characters on the right of the first line. They start with MEX, but when I enter this, it says it's invalid, because it has letters. If I enter just the numbers, it also says it's invalid.
There's a checkbox that says "I don't not know", which I can check to continue with the application. But I'm not sure it's a good idea to continue like that.
Here's a BCC sample
It's for Mexican citizens
Thanks a lot for your time!