There was a reinforcement plate and a pad on each side.

http://www.1954advance-design.com/DirectionalSignalsInstall.htm

http://www.1954advance-design.com/Documents/Signal%20Install%2009.pdf

Heed the grounding instructions - use a conductive anti-oxidant copper-impregnated (non-silicone-containing) grease on the clean bare holes in the body side panels.

If you have lamps that have mounting posts that are off the holes in the template, you might have older auto lamps that look similar to the 47-55st panel-body lamps? My understanding is that there were also other similar auto lamps with mounting posts orientated 90 degrees differently.