I have been watching alot of old westerns lately and am wondering why the cavalry or regular army worn a flap holster with the butt of the pistol facing forward.  Was there some advantage to this??

It always seems to be on the right side...I am not sure if they ever used it in a crossdraw manner.  Always seemed strange to me though and no one has ever had an answer for me.

The US Army at that time did not acknowledge the existence of left-handed people.  A cavalryman wore a saber on the left hip to be drawn by the right hand.  The right side was the only logical place left for a pistol holster.  The holster was canted forward to reduce the likelihood of accidentally dropping your revolver while riding.  It was set butt forward so it could be drawn by the left hand.