There is a trader that gets around the vintage shows who in addition to selling new tools also sells good quality old spanners. He has a very good knowledge of the older stuff. If I see him and can remember to ask I will.
I wonder if it would be worth asking the Tool and Trades History Society (TATHS)
I think that will be a Mayfield 10, the Museum of English Rural Life at Reading have the factory records and should be able to date the machine from its serial number.
For equivalent tyre size there are a number of online conversion calculators
I think it is worth noting that I think this topic refers to the split wheels as used on Monrotiller etc, not the split rim type of wheel as used on commercial vehicles, which can be lethal.
I would say it is from somewhere in Europe, France, Germany etc. Possibly Bungartz, Siemens, Stuab. It might be worth having a look through Michiel Hooijberg’s website