woreq123, What I know is ...
when you go for stamping, officer takes finger print of index finger on the form, I think form is called I89. This form has to go back to service center to get the plastic card ordered. The local INS offices takes their own time in returning these I89 form to service centers, that is why you see some people in same service center receive cards faster. Once the service center receive this form, cards are ordered for printing. cards get mailed to you from the place where they are printed.