Mark,
About two years ago I was also looking for the blue nuts. I went to my MB parts guy and he ordered the "new" nuts. They are about the same in size and looks except, they are made of metal, but do work just as well. The reason for the plastic nuts is so you turn with your thumb as to not torque the nut too muck. If you were to put a wrench on the nut, you would tighten too much and pull the stub bolt out of the wood. But please do try the "new" metal nuts. I will look tonight to see if I can find the sales recept to get parts number.
Bob