They were any Model 14 or K-38 Masterpiece that were either special ordered as single action only, or converted after purchase. The only difference between these and standard guns is the double-action sear has been removed, or never installed. Factory single actions had a hammer that didn't have the slot for the sear cut in the front of the hammer.
It is impossible to differentiate between a factory gun and one that was converted later, if the conversion was done by installing a factory single-action hammer instead of by removing the sear. Functionally they are identical.