ride shotgun

[Joan Houston Hall]

DARE is into the R's and S's now, and we've been looking at the phrases
with "ride."  In considering the phrase "ride shotgun," meaning 'to ride in
the front passenger seat of a car,' we've found that it's widespread
throughout the country, and that there are more than 65 web pages that
offer "Shotgun Rules!"  But we got to wondering about the presumed source
of that meaning, 'to ride as an armed guard on a stagecoach,' and have come
up with nothing.  The earliest quote we've found is from 1961, and is for
the car sense, though it alludes to a historic sense.  Is the stagecoach
meaning a figment of our imagination?  Did it come from a popular Western
movie and get quickly transferred to a car?  Can anyone help?  Thanks!