I was planning on buying a 638 or a 642 no-lock. I couldn't find any info on the 638, so I emailed Smith and Wesson to ask them if they plan on offering a no-lock version of the 638. They responded saying:
sorry no longer have non-lock guns
I don't know if I'm going to buy a j-frame if I can't get one without a lock, which really sucks because I was looking forward to trying pocket carry.