according to the bash manual the following key binding should work: "\M-p": history-search-backward what happens is that instead of getting a completion from history i get a ":" character. the same key binding works on a second box (same base setup except for the locale stuff which on the second box has not been set to anything at all).
Don't restrict bugs without any reason please, the checkboxes are not there for fun.
you forgot to post `emerge info` from both settings as well as the locale settings and terminals in use