Richard, the Census List shows that the Federal 20 ton 6 x4-4 Semi-Trailer rig was to S/M 2149...Pearsons assembled the Federals and I have a photo of one. Census Numbers L 4919986 to 4920435. However note no 'H' prefix! The trailers were to the same Contract S/M 2149 and allocated X 5847938 to 5848387. 450 units each. However 'Trailer 20 ton 8 wheel (4 twin)' was 'Separated from Contract S/M 2149'. However I think I made a mis-judgement...I assumed that this back end of the truck below was a Diamond T 980/981 but the fifth-wheel arrangement on the DTs seems to differ.
It is in fact a Diamond T Census Number H 5292557 to S/M 2147. It appears that the Trailmobile trailers were originally for use with the Federal 604 tractors , and then the trailers were split off from the contract and then used with converted Diamoind T 980/981 tractors,
Note the semi-trailer above was 'assembled' by Pearsons in Liverpool and the previous one was 'assembled' by Lep Transport in Goole. Both companies assembled PKD Diamond T 980/981 and 968/969 6 x 4 trucks!