Not strictly conversions - it was the buses that were the conversions - were the experimental gas trailers used for a short time in mid WW2 as an attempt - not a successful one it seems - to save petrol. U Boats were sinking many oil tankers especially of the Atlantic coast of the USA.
The BCT had these gas producing trailers, based at Birchfield Road which paired with the piano front AEC Regents of 1930/31.
There is mention and photographs of these in other BHF threads.
The Midland Red also were lumbered with gas producing trailers. It was a government directive.
It seems they were used with 15 of the the single deck SOS IM6's in the Evesham area.
Looking at a photograph of the rear of one of the trailers used by BCT suggests that there was a T plate (for trailer); the customary clear reflective jewels on a black background.
The B&MMO one did not have the T but did carry a registration plate being the same notation of the bus towing it.
Whether this applied to all the trailers I do not know. There were some difference in appearance of the firebox between the two photos, but maybe they were from different manufacturers.
