David,
I have 2 of these '4.2" Mortar Trailers'. One is immaculately restored and the other I am starting to restore. My unrestored trailer is timbered in Elm and I have been told by my late Father-in-law, who spent most of his in the Timber Industry, that Ash would be a good substitute.
I call these Mortar Trailers as the GS type has rounded wheel arches and a shorter body sitting back on the chassis. I have many photos of them being used as GS trailers during the end of WW2 and postwar. I believe that they may have been favoured to the GS Trailer due to a slightly larger load area and without the top slats the load may be slightly better protected.
Regards,
Les.