Head Ski Company was the last series of the Wing line
Bob Lee sold out to AMF, who later sold to Head Ski. I think you are right on with the year of 1970, although it may have been a bit later. We lost our Wing expert when WINGNUTS passed away. He know more about Wing bows than anyone I know of.
I have a Thunderbird in my collection. It is a good shooter if you like a short bow.