This question comes up surprisingly often, and I'm going to guess the OP is asking about K38's for sale listed as single action only (SAO). In the majority of these the double action was not "deactivated". S&W sold guns from the factory in SAO configuration, and also sold the hammer/sear assembly seperately. As far as I know the only way to know if one came directly with that option is with a letter, or the original box.

For bullseye shooting this made the hammer throw slightly shorter, and it was easier to cock, both important in a long day of precision competition. It doesn't sound like much until you try it. :-)

There are a lot of past threads on this, and I'm sure there are better descritions than what I've been able to recall today.

I loooooove mine.