The A plate is what they are calling the spacer plate. When I purchased my Cuddlebug several years ago, I read somewhere that the embossing mat could be used for a C plate. I didn't get a C plate, just an embossing mat. Made a note with Sharpie on the mat that it could be used as the C plate. Just don't have the source.
Hope this helps.
I'm delighted to find replacement B plates.