could be a custom design for a gravel driveway, or possibly a dealer installed item for the gravel driveway use. Having well over 1800 foot gravel driveway I can tell you I wish new Holland design wheels for mine do have pictures of it?
The wheels are added by the end user or the shop that sold the snow caster.
You can find great pnuematic swiveling caster wheels with ball bearings at Mcmaster.com
All that is needed is some angle iron and a drill for mounting the angle iron.
You need to purchase nylock nuts or use loctite to be be sure nothing vibrates loose when you attach everything to the snow caster housing.
The wheels prevent the corners of the snow caster from digging into soft ground and allow easier turns with less wieght on the snow caster whether in front or in the rear.
The4 skids can be left on the snow caster if desired as the tires usually keep the snow caster less than an inch above the ground when working.