I did a little looking around on the Madeira website and I couldn't find a No. 30 cotton thread but did find a No. 30 rayon thread.
This was the best website I could find explaining why a thread could be No. 30 and 50/2:
http://www.fashion-incubator.com/archive/thread_sizes/
I don't understand it all, but it does seem that the No. 30 is a thread weight.