In response to G.J. Kralis and his/her problem with your "illegal plates", if I were you I would take one of your "warnings" to the BMV and show them and tell them how many times you have been stopped for the same offense. If they insist they are legal plates, I would have whomever tells you that to write you a signed note stating that fact.
With the problems you are having I would demand one and keep it in my glovebox for the next "know it all." Should you get stopped and ticketed after showing them the signed note, I would take it to court and file harassment charges and also for attorney fees.
It also wouldn't hurt for you to call the State BMV and give them all the information on your pick up and the plates that were given to you and ask them! If they are not the correct plates, I would insist the BMV give you the proper plates by adding the added fee and deduction for "pro-rated" plates. Good luck!