Hi Richard,
The right hand tank in the 1st picture on the other thread is the correct drain.
The right hand tank on the truck should be a stand pipe with a 90 degree turn with thread for 1/4 BSP pipe nut.
The left hand tank should be a stand pipe with a push pull tap.
There should be a push pull switch over tap on the right hand chassis rail just behind the cab.
I will try to post some pictures over the week end.
Peter.