[aerohead]

1) For $36 I added a passenger side mirror to my 1984 CRX.
2) At that time, it was quite common for automobiles to have only a single mirror.
3) My Rambler American had single mirror.
4) My Morris Minor 1000 had single mirror.
5) El Camino had single.
6) My Karmann-Ghias had single mirror.
7) VW Transporter had two.
8) Dodge D-100 had two aftermarket mirrors.
9) T-100 has two.
Some 'historian' may be able to peg a date when the DOT/ NHTSA began to 'mandate' the second mirror.