Why don't some of the substitute labor people call INS customer support and ask what their priority date is ? Once you get the answer, please post here. There seem to be confusion on their priority date. Some people say that it is effective the original SESA (Labor) date and some say that it is the I140 receipt date. I am inclined to think (seems to have read some where) that for substitute labor cases, PD is RD of I140.
Note : First of all, check if PD is mentioned in your I485 receipt. For normal cases, they mention the PD as the Receipt Date of Labor certification.