Colin,
Sorry, I cannot find the parts book, but it is the standard tail light used on most British vehicles during the war, cylindrical body, about 1" diam. with red lens, this is fitted on the offside rear corner of the trailer. There is a Lucas changeover switch, which I think, was mounted on the rear cross member, you do see new ones on the stalls at shows, they are black plastic, round, about 2.5" diam, with a lever out of the side, they switch from tail to convoy. The convoy lamp is again the standard item, mounted under the body shining on to a white plate clamped to the axle.
Richard