Hi John,
My thoughts are that the original low-line version was done as you say, for airborne reasons, and with a low centre of gravity would have handled better when towed I guess ....... but for practical reasons such as maintenance after the war, it may well have had the generator unit raised. Only way to find out would be to locate documents for this piece of equipment such as modification instruction. They would be in an EMER Eng. & Misc. I think. As REME Museum has recently moved, I don't know if they still have comprehensive library of EMER's from the distant past. What they might have, being one of their bits of kit, would be photos of them in use and that might give a timeline on when this was done. Another point, it might be from a different contract to your other Welding trailer?
regards, Richard