They are 400/60x15.5, I got them as complete assemblies from the British rubber company,
https://britishrubberco.co.uk
The rims where a standard stud pattern and the standard offset worked in that the tyres are the same width as the blade, and the overall diameter was about the same as what came off.
I would of liked a little wider but getting rims with a different offset to keep the width right was the problem, also I was struggling to find anything to keep the overall diameter similar to oe.
I tried to get these in 18 ply thinking they would be a bit stiffer when working FoW but where un available at the time, I could only get 14 ply, the OE tyres where 10ply I think.