Keep an eye on Ebay, Colaws RV Salvage, the classifieds here, and Craigslist- one will eventually turn up.
I have seen the Monarch lights for later 70s on ebay, but not lights for 71. A few other years should also use the same design. Colaws phone # is 877 548 2125. In the case of these tailights I don't think left or right matters just turn them upside down. The not so good thing is I think to mount these tailights you have to take the bathroom half apart.
If you ever get some extra lenses I could use a spare.
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Pay no attention to the man behind the curtain- WoZ
My spare lights are for sale @40 for 1 or 70 for both. Mwells has first right of refusal since they were the first to contact me about them. I need to try to recover some $$ from a stupid purchase.
The PO took off the rear lights of the TT and now they can only find one. I have the one on the door side (passenger side of the TV)
If you can't find one - could you live with the late 70's square lens ones. If so, you could build them from parts still available from Airstream. They have the frames (plastic is all I saw in the factory store) and cans - the Monarch lenses are available from Airstream & RV dealers and the on-line vendors here. Out of Doors Mart has the lens gaskets and others may also....
We have the square frames on our 71 (done at Airstream by the PO) and I've never had anyone notice unless I point it out.... just another option for you.