PowerEdge R300 Memory Modules 5300E vs 5300P ?

Hi all,

I bought a pair of Crucial DDR2 667Mhz CCL5 ECC memory modules for my PE R300. They just don't work at all. I noticed the label on the modules reads "5300E" while the modules already  installed in the server read "5300P". Can't those "5300E" modules work in my PE R300 ?

Thanks for your help!