Give the Superior Threads website a try at www.superiorthreads.com. Click on the "education button" and then "reference guides". Ther is a ton of thread information here and a chart showing which of the threads they sell are 50/3. The chart only shows their products, but if you're stuck for thread they are a really good resource.
You are absolutely correct about the hand and machine quilting threads. The hand quilting threads are "glaced" or "glazed" and has a finish on them that can make sewing machines very unhappy.
If this is a double post, my apologies. The internet is acting a little balky!