A friend of mine has this sticker in the back window of his recently acquired 69 Mustang coupe. The car spent all its life in California and it came to Europe with that sticker.
Okay, I showed it to the wife and she said, 'Yeah, you had one of those on the Mach 1'. Then it hit me. It's from a company called 'Tuneup Masters'. They did low cost tune ups. It was all they did. (Kind of a new concept at the time). If you had the sticker and were stranded on the road side a 'Tuneup Masters' employee was pledged to stop and help.
I was going to say it looked like a "Tune Up Masters" decal to me - I used to see lots of cars with those decals in Salt Lake City when I was in high-school (mid-'80s) as well.