It all depends on the airline. Some accept the stamp, others do not.
On contacting the German consulate I was told that a transit visa is required.
If you have a not-expired stamp on ur passport (H1B or Canadian visa) you do not need a transit visa.
In my case I had the 1-551 stamp, plus a valid H1B plus a Canadian visa. I flew Delta from India and I was asked if I am travelling on the I-551 stamp or the H1B. I said H1B. That guy would not have had any complaints if I had told him I-551 stamp.