FED:-
Being a cynic I think that that might be an old wives tale.
While I cannot admit to first hand knowledge (this and glider towing, for which the release ring could still be seen in the tail lamp cluster, were mercifully no longer a part of the OCU syllabus at Thorney Island in 1963), I have googled this pic of at least one Jeep/Land Rover (and gun?) underslung on a Hastings in flight. I offer it as Exhibit A m'lud:-
