The Motley mount was used for AA. There are a few versions : MkI till MkIV have only one Bren LMG on it. Because more firepower was needed these mount were adjusted so they could hold two Bren LMG (the Mk1A till Mk IVA.
A new mount MkV was also developed for two Bren LMG.
The mount were indeed for 15cwt Trucks such as the Bedford, but some were mounted in Light utility Trucks like the Austin, Hillman and Morris.
More information can be found in : "Small Arms Training Volume I, Pamplet No. 6 (G.S. Publication 936, 1943)"
The picture shows (I think) a Motley Mount Mk1A. If it´s possible I would love to have that picture for an article I writing about the Motley.
Cheers,
Arjan van der Hoek