it's the brainchild of the PO, and no it's not standard - originally it woulda had a rotten tar impregnated fibre underlay below the mat that was the primary cause of rotting floors because of the moisture it collected and held - folks here seem to have agreed that a stuff called Peel and Seal is the best modern replacement, but you can get the repro original stuff

Bill

Moved over to the Passing Lane

"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics