@HarryT: Nope, the sort order changed. (It's was already different between the K3 & the Touch, but it's even different from the Touch on the PW...).
The underscore happened to be near the top of the list both on the 3 & the Touch, so it was indeed a good choice, but not anymore ;/.
Code:
# Kindle ignores these characters and sorts them randomly (unless name is only invisible characters:!
SORT_INVISIBLE = u'-?/]#.\'\\*})&:@%;"!,' # means the visible are " _`^~'([{$+<=>|" + 0-9, a-z
# And on the Touch... Didn't care to test the 1 char invisible stuff, so the order might not be completely accurate...
TOUCH_SORT_INVISIBLE = u' #[:?)\\"{.!/%…;&]},*\'(@'
# And on the PaperWhite... (Yeah, completely different than on the Touch... Oh, joy...)
PW_SORT_INVISIBLE = u' _·™…].¢#*`)"¬£-;@&{^/,©%~=|®}€>[($+:?!¥\\<§'
# Sort order for all chars (invisible needed in case the collection is 1 char long)
SORT_CHARACTERS = u'- _,;:!?/.`^~\'"()[]{}@$*&#%+<=>|0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
# And on the Touch... (Note that you shouldn't be able to put a leading/trailing whitespace in a collection name,
# be it via the default UI or Collections Manager, that's why I put it in the invisible list on the Touch).
TOUCH_SORT_CHARACTERS = u'_-`^§©®+<=>¬|~¢$£¥€0123456789 abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ#[:·?)\\"{.!/%…;&]},*\'(@'
# NOTE: @ possibly sorted like A ™ possibly sorted like tm or TM
# PaperWhite... Don't have a lot of choices anymore to put stuff in front... A single quote, and that's it :/.
# The case sorting interacts with the invisible chars in a weird way, some of them sit between lowercase and uppercase if they precede the exact same letter...
PW_SORT_CHARACTERS = u'\'012345789 abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_·™…].¢#*`)"¬£-;@&{^/,©%~=|®}€>[($+:?!¥\\<§'