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#51
You are referring to Builder's OEM which are different than Manufacturer's OEM, except that they both become locked to hardware.
One is retail sold to home computer builders to complete their builds. In the past, shops would sell them to someone who bought any other hardware item. Now they are pretty much sold like retail.
Manufacturer's OEM are sold in bulk to computer makers and have batch keys on their stickers which generate a unique key when locked to the mobo. You can detect this unique key by running Belarc Advisor, or else during a reinstall reactivation will be elevated to a robocall where you will trade a series of numbers which allow MS to sort out your unique key from the batch one.