[Richard Coutts-Smith]

Thanks Alan, had discounted the louvers as being something in between the tank and the truck, not the look I had imagined at all.
Hi-jacking of post resolved, and my thanks to David for such an interesting post, it appears that there was a lapse in the amount of red tape back in the day, tanks too high, build new wagons .... Job Done.
I do wonder what would have been the solution if the tanks had been too wide.
Rich.