Bump codes are the little bumps on the bottom of the bag, I found these (courtesy of a user on Flickr)
Codes for Homer, Bart, Marge and Lisa
https://www.flickr.com/photos/el_gra...-naTbBt-bmW1BE
Milhouse, Ralph, Apu and Nelson
https://www.flickr.com/photos/el_gra...naTbBt-bmW1BE/
Maggie, Grandpa Simpson, Ned, Krusty
https://www.flickr.com/photos/el_gra...naTbBt-bmW1BE/
And finally Itchy, Scratchy, Chief Wiggum and Mr. Burns
https://www.flickr.com/photos/el_gra...naTbBt-bmW1BE/
Enjoy!
