It's exactly that: just a stamp. It says " Processed for I-551. Temporary evidence of lawful admission for permanent residence valid until (1 year). Employment authorized.
The officer will then write his/her initials and the date underneath, as well as your alien number, the BCIS center initials and other small codes related to IBIS check, etc.
To be honest, the thing doesnt look very "official" or anything... but hey it does the trick, i guess