Assuming the error was produced for the entire duration of printing 1st edition Jungle sets, I’d say 1/7 (so ~14.3%) is a “d” edition error of all English 1st edition Jungle Butterfrees, since there are seven Butterfrees on a Jungle Uncommon sheet and the error would be onone of those seven positions:
Image from @garyis2000 , which he posted here on my request (thanks again Gary 
).
Greetz,
Quuador