You could say ,
'upon further investigation I found out this mirror was installed as a dealer fitted item' therefore it did leave the dealer as a twin mirrored car. if you search a little more you may find the matching drivers side mirror that was fitted when the car rolled off the showroom floor.'
explain the fact that while it may not have rolled out of the factory that way, it rolled off the showroom floor that way. so as far as you can say it left the pantera dealer that way.
maybe that helps.
send him to someplaces that MAY have matching mirror that is real expensive !!
good luck !
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