The only logical explanation to this would be that the A# and the purported VISA NUMBER during interview are entirely different.
Meaning, if your A# is 99 999 999, then during your interview, you may get a VISA NUMBER which is something like, 99999 99999 99999, as somebody says it is a 15-digit number?
In that case, that would make sense as to what we are hearing from others. If not, the mystery continues. That is to say, if the A# and the VISA NUMBER are going to be the same, then there is no reason why they would say, "we don't have the visa number available for you right now and so cannot stamp it," because without A#, the guy could not have been called for an interview. Right? Am thinking out loud here. Well, that doesn't explain another problem wherein as per the SOP of INS manual, cases are to be transferred to the local office for interview if A#s cannot be generated in the system!!!
Phew!!! I am lost