Your Trusty Steed is a Mk2; NTS stands for “New Trusty Steed”.
No. 225 was sold to K.L.Distributors, the Australian importers, on 17th January, 1952 and was fitted with a Norton Big Four engine, 600cc side valve with engine number 32KK.
If you post some pictures of it here I may be able to tell you more by looking at it.
Best of luck with it.