T 21 Ballast
When we started AE flying on a (tasked) regular basis many of the Cadets were quite young and 'light'. Answer to this situation was a spare T21 seat bottom (held in by dot fastner) to which had been bolted a thick slab of lead suitably curved to fit. (Gold was too expensive)
I suspect that the Woolwich Arsenal was the supplier of this 'mod ' as they provided most of the materials for our home built T31 trolleys.