Seems obvious β a key is a key. The trick is to order the right thing. I think I got lucky because my old drill recently lost its key. Great news - the key fits perfectly. The "key" to your success is ordering the right key. Finally I measured the diameter and everything worked out, or maybe I got lucky!
Perfect for this very old Jacobs chuck from the early 1940's. Dad bought us kids an Atlas lathe (an old Sears Roebuck) just before the war, and sure enough, when he got home one of us had lost it. It was a real pleasure to receive this new wrench that fits perfectly with this 3/4th century cartridge.
If you've got an imported Jacobs cartridge with a chewed, almost useless, and potentially dangerous key, this could be your lifeline. I have an MT2 1-13mm cartridge and the local hardware store didn't have a proper wrench (and they have everything!). I bet on it and won. No more trapped hands or bruised joints!