NDT tyres with the squared shoulder were often known as mud and snow types compared to NDCC which had a rounded profile for cross country use.
Strangely most of the Goodyear 15.00-20 as fitted to Martians and Militants were NDT whereas the 14.00-20 on Scammells and Militants was usually NDCC, check out Rob's Militant tanker on 15.00-20 which clearly has the squared shoulder.
A Staghound weighs around 14 tons so would need the 18 ply as suggested. Last time I looked into buying new 14.00-20s they were circa £700 each and that was about 30 years ago, make sure you are sitting down when you get a quote for new ones now.