It sounds like that one might have been assembled with the wrong rear plate if it's got 3 brushes, but that doesn't seem to be possible if there are also brush holders in the case. Pictures would be good in this situation!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!