Hi Ian
Clives post is spot on - the variation in mineral wagons from the steam age - wooden, slope sided, cupboard door - had all gone by 67. SO for late 60s early 70s as Clive says its the variations on the 16 tonners for us - with 21 tonners on some flows
If you go back up this thread there are some prototype shots of typical trains of the period if you haven't already seen them
Cheers
Phil