I thought you'd know Karen, thank you! I have heard picas mentioned, but never really thought about it - it's still confusing, though. It seems that 60 on the 2P line is equivalent to 15 on the 48P line - and 40 on the 10P line is the same as 50 on the 8P line. Hmm! I'll figure it out tomorrow, when I'm not sleepy.

Btw, John used to be a draftsman (mechanical engineer) before getting into photography and later illustrations. So it must be from that period.