That made me curious, so I checked the one on my Toshiba laptop running W8.1, and it too is only 20 digits long. Ends with AAOEM.
I’m not sure now what i tried for the serial key- i found the actual product key, and it is 25 letters and numbers- the number i first tried, with the 20 numbers, had ‘oem’ as the second set of letters- but my actual product key does have 25 numbers- liek it’s supposed to-