Round stems are by far the simplest stem design around. It is literally a piece of round stock riveted into the kingpin of the caster. They actually are not that different from a grip ring stem. Instead of using a locking ring to hold it in place, it is a simple compression fit.
But, they are very rare. Normally a grip ring or threaded stem is used due to their positive locking. We do not recommend using a round stem, if at all possible. Contact us to discuss your options.