The No Value is one idea. The Forever Good for First Ounce of FCM is a newer idea.
Yes...a friend of mine sent me a block when they first came out in the late 80s. He didn't explain and I wasn't a serious collector, so I didn't know what IST meant. He later sent me a 2ND, and I got it. I originally thought the ST meant "Pound Sterling" and wondered why it didn't just say £I.00 like my others!