I'd bet on that tank being military production.
The illustrations in Truck Data Books might not have been updated properly by GM every year, but, assuming that they were, here are some illustrations from USA Chevrolet Truck Data Books:
1946 pre-Advance-Design - OVERFLOW RETURN TANK (supposedly available since 1937)
1947 Advance-Design - OVERFLOW RETURN TANK
1949 Advance-Design - notice the change in the name of tank "OVERFLOW CONDENSER"
