Not necessarily a transcription error. In early printing the letters eth (which is what 999 is describing) and thorn were often replaced with D or Y - hence "ye" as in "Ye Olde Tea Shoppe" actually represents "the", with the y replacing þ to represent the "th" sound. These letters are still used in modern Icelandic to represent voiced and unvoiced "th" sounds.

So "Thou" might have been written "Đou" which was then printed as "Dou"