OK - I'm having an old geezer moment.....
I'm looking at the score for Mussorgsky's Night On A Bald/Bare Mountain.
The trumpet part is marked as "Trombe in B", but on the first page of the score there is no key signature (piece is in one flat concert) and then at measure 9 without any other key change, the trumpet part is marked with one flat.
The (F) horn also starts with no key signature and changes to one flat at the same point as the trumpet.
However, the Bb Clarinet stays in one sharp throughout (as I would expect it to).
Which particular nugget of common musical sense am I missing here to make me not understand the above?