Have both apportioned plates been lost or stolen?
I believe they were stolen, yes.
If both apportioned plates are lost or stolen, get from Report of Lost or Stolen Motor Vehicle Items MV-78B from a police agency.  Also get the same form if your items were lost because of a crime.  Your items will be replaced at no fee if the form shows the items were lost because of a crime.
How do I apply to replace IPR registration items at the DMV?