The Selectric didn't really do rollover, but it had a mechanism that felt like it. Each key lever had a small tab that entered a trough of ball bearings that ha

The Selectric didn't really do rollover, but it had a mechanism that felt like it. Each key lever had a small tab that entered a trough of ball bearings that had just enough slack for one tab. If you pressed a second key, it would displace the balls and descend when the previous key withdrew.