Wheel studs were changed in the last year of Series 2a production from 9/16" BSF to 16mm, almost certainly as part of their gradual move to metrication. These were already in use on the Rangrover just going into production.
Interestingly, there was a short period when they were supplied with the earlier studs but with nuts whose outside dimensions match the 16mm nuts (27mm).
There is no specific date for the change, and it probably varied depending where the vehicle was built, but all Series 3 Landrovers left the factory with the larger studs. However, since assemblies are so easy to swap, no bets on what is actually fitted to any particular vehicle today!

