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.
The teletype keyboard mechanism was less sophisticated than a Selectric. Also, I seem to remember it took like a pound of pressure just to push a key on a teletype, so you ended up vigorously punching it with your index fingers. A Selectric is like typing on air compared to a teletype.
