Jewelry Maker's Square in Stainless Steel
I have used one of these for over 30 years, and have not seen them anywhere.
If you do not lose it, it will last you a lifetime.
Good for marking 90 degree angles, making square settings and square pieces, marking straight lines, etc.
I keep mine handy, and use it many times a week.
Many people drill a small hole in it and hang it on their bench.
It measures 32mm long, 21mm tall, and it is made out of 24 Gauge Stainless Steel (0.52mm)