Taillight Lens Rehabilitation Part I

I decided to rebuild something really cheap and easy to restore or upgrade.  I have euro-style amber lenses for my taillights, brand new, but the housings are used and in need of some care.

Here are the two taillight housings:

Left side, but notice an extra bulb socket in the upper big reflector opening as compared to the right side below.

Right side.

I plan to paint these, but why are there two bulb sockets in the left housing?  The second socket is definitely an addition, given the crimped wires on the back and the recessed socket.