What style of lens is on yours? Amerigo simply used what was available thru RV suppliers at the time so it's nothing specific to only amerigo units.
My FG16 has Pathfinder lens on it, I've added a couple of pics for reference.
I found some spares for mine on ebay years ago & they do pop up now and then as old stock gets discovered/cleared out.
Another place to look is salvage off of old RV's from that era. I had seen an old early 70's Winnebago motorhome & it had the exact same tail lights
that my Amerigo has.
Here is a recent ebay auction for some NOS ones, the same seller had 2 pairs that they sold
. Maybe you can try to contact them to see if they have any more left?