just received a letter of suspension of registration that is no longer valid for car. what should I do?

So, what had happened was I let my insurance coverage lapse longer than 30 days (pa st driver btw), and rather than providing the proof of insurance to the dmv, I got re-insured right away. Then about 15 days or so later, I transferred my insurance and plate from this particular vehicle to another one. Had no issues in doing so and this new vehicle is completely registered and insured with the old plate. Then tonight, I got the suspension letter stating that they want me to turn in the plate from the old vehicle for surrender, however, there is no license plate number indicated for the old car. It is just left blank. Since the plate is registered to another vehicle and insurance is current on that vehicle, how am I supposed to turn in a plate for a vehicle that is no longer registered? As stated earlier, the old plate is on another car, and on the suspension order, they don't list a plate number for the old vehicle in question. Am I in the middle of a catch 22? Any information would be greatly appreciated so I can get this taken care of. Thanks in advance.