Suppressing key signature for transposed parts

[Nick Didkovsky]

Hello Lilypond users

I want to extract a Bb trumpet part from a concert score. The score is 
not really in a key signature (atonal music, not c major), so I do not 
want a key signature in the extracted part.
I see that as long as no key is specified in the music file, then no key 
signature is introduced in the transposed part.
However, the transposed part then mixes sharps and flats despite the 
command which according to 6.1.8 in the docs ought to print only flats.
\transpose bes c' \new Staff \jjfStaffAA

For example E is transposed to F# instead of Gb

If I introduce "\key c \major" to the music file then the transposed 
part is spelled using only flats (desired), but a key signature of D 
major is introduced (not desired)

It looks like suppressing the key signature makes Lilypond spell with 
sharps all those transposed notes that would otherwise be sharp in the D 
major key signature.

Can someone advise how I can both control accidental spelling and 
suppress key signatures in transposed parts?

Thank-you
Nick Didkovsky