what does 2Rx8 mean? in the 2GB 2Rx8 PC3-8500s 07-10-F2

Intrepid 223

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Dec 5, 2013
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Hello All
I am upgrading the memory in my mid 2010 mac book pro. from 4GB to 8GB
Apple says I need a pair of 4GB DDR3 PC3-8500 SODIMM 1066MHz
What came out of the machine is 2GB 2Rx8 PC3-8500s 07-10-F2.
After I install the 4GB DDR3 PC3-8500 SODIMM 1066MH, When I fire up the machine I get a single beep indicating no ram installed.
When I reinstall the old ram the machine fire's up smartly.
This is the second set of ram I have tried, I don't know what 2Rx8 means.
Thank you in advance for your help
Dave