Ok, so...
1. in 66 the deluxe belts were a separate option to the pony interior, and they came with the warning light.
And
2. no way to tell what belts came from the factory based on the door tag showing standard or deluxe interior.
Would assume one could deduce if a mounting hole for the warning light exist (where was it mounted under the dash, and any specific D or O size hole to look for?) that the deluxe belts "may" have been original, an indicator but not proof. What other tell-tales could one use to attempt to determine which belts came with the car, or is it a guess?
Helping the shop that does my body/paint do up a 66 T code coupe and am in unknown territory as I am more familiar with 68s. Any tips to help the owner figure out what may have been original appreciated.
Will look for any under dash hints and stray wiring leads that may provide some info... the car had the interior replaced and dash repainted in a prior life so it has been "adjusted"...