shell porcelain sign mystery

Hey Guys, I got lucky to bought recently an unknow shell porcelain sign, but what was it going on ? I never seen another one before and never seen on old pictures of gas station

the sign is written in french translated by "shell oils make engines last" I think it was used in Belgium ( or france )

each parts are pretty narrow 5,11 inches ( 13 cm ) by 43,30 inches ( 110 cm ) the 2 parts are the exact same sizes and there are 8 holes at the exact same places on the 2 parts, the 2 signs are not cutout from a larger sign as the porcelain is neat on all edges

I'm not sure if the 2 parts were meant to be placed side by side or on top of each other

I have some ideas on where it was used but nothing confirmed

- on the side of a shell delivery truck
- on the tailgate of a shell delivery truck
- on the marquee of a gas station
- on the side of a race track as an advertising

anybody has an idea ? or black and white picture of this sign in situation

Thanks