Comparing the first 7 MB of the fx-CG10 flash (0001FFB0:2010.0910.1720) to a fx-CG20 (0001FFB0:2010.0916.0917) doesn't show many differences. Apart from the calc-id (0001FFD0) and the BIOS checksum (0001FFFC), only the bytes at 0x303 and 0x305 are different.
fx-CG10: 0x303 : 0x41
fx-CG10: 0x305 : 0x5A
fx-CG20: 0x303 : 0x44
fx-CG20: 0x305 : 0xAA
Of course it is possible, that the hardware differs and the methods to distinguish between CG10 and CG20 is not only based on 0x303 and 0x305.
At present, no other differences are known.