Join Date: Jun 2004
Location: Northern Nevada
I was going to guess mid-'80s. I had the same stem, or one that looked just like it, on my first mountain bike, an '84 (or so) Mongoose.
One thing to watch for: The bars were too close on the 'Goose, and I always wanted to put on a longer stem. But the quill diameter was .833 inch (the old BMX/cruiser standard, about 21.15mm) instead of the English/Japanese 22.2mm that became much more common. I never did find a stem.
That 22.2, by the way, is commonly called "one inch," even though an inch is actually 25.4mm. Who knows why?