Just an update on this -- I sent the 19 and 18mm 1/2" drive sockets to SK. I added a letter stating that I didn't care about a warranty replacement, but that they should look at these carefully to see why they failed. I also mentioned that I had broken other sockets (1/4" drive and 3/8" drive from sets in this era, but never bothered to warranty them).
Surprisingly, they sent back a "Thank You!" letter, and new 18mm and 19mm 1/2" drive replacements. Great customer service; I wasn't expecting that, but Ideal Industries has changed SK for the better. I will purchase "new" SK if I need sockets in the future.