This type of hard rubber wheel caster can be found on older Wurlitzer models. 
         
I am calling the casters offered in this listing Type P. 
This is a 1.5 inch diameter hard rubber wheel with a shank that is about 1.5 inches long and 7/16 of an inch in diameter.
Metric equivalents are 3.81 cm wheel  with a 3.81 cm long shank that is 1.1 cm in diameter.

It has not been cleaned - there may be some rust on the remaining few I have of this type.