回複：請教小白兔和其他專家: EB-1b RFE question

It really doesn't matter when, if the date was earlier than your I140 petition. What matters is if you really have a permanent job. Based on my experience, post-doc is absolutely not a permanent job. You should have a tenue-track position for professor or "permanent" position for researcher. I believe in your case, you need a letter saying that you have a research position, i.e., research associate, with an indefinite term. Generally, no one will write in letter that you are offered with a permanent position.