Hello Charlie.
As mentioned by Dave 6.00×22 have not been made for 30+ years, I tried to locate the whereabouts of moulds several years ago, but it seems they have been destroyed/lost.
Apart from Plowmate, Iron Horse, they also fit David Brown 2D, Coleby shire, Garner 2 wheeler and 4 wheeler, Gunsmith et etc.
Most surviving tyres are now showing their age with cracked sidewalls etc, most important to shield them from the sun.
Chris