It is very possible it was a screw up like you mentioned.
If it's more of a greenish gray color then it would be under body primer.
This is the factory under body color: DPLF 40 Green/Gray from PPG.
The cowl, firewall, jams and all under body parts were this color. But the exterior sheet metal had a red-oxide over the underbody primer, fenders, hood, bed, back of cab, doors, etc were this color.
Mark D.
1968 W200 Sweptline
1969 D100 Utiline
1971 D100 SE Sweptline
1993 D250 Club Cab Sweptline
1986 W250 Cab & Chassis
1978 D300 Sweptline
E. Brookfield, Massachusetts