Electro String was the former name of Rickenbacker. Normally they made horseshoe pickups. But that is a model similar to the high gains like on the 4001 bass bridge pickup.
 Rickenbacker did make some later instruments with the Electro name.'
It would be possible to describe everything scientifically, but it would make no sense; it would be without meaning, as if you described a Beethoven symphony as a variation of wave pressure. — Albert Einstein