Hard Mount.
I believe the spin speed was limited because the washer had no suspension. With the hard mount design, all of the G-Forces were transferred to the building. Too much, and things could REALLY start shaking and bouncing. If it was too much, the washer could eventually tear itself free from its mountings and go on one heck of a dance excursion.
Once the technology for a soft-mount machine was developed suitable for a residential / less-than-perfect installation, spin could increase. Also, the Bendix washers have small-diameter drums, so the G-forces are extra low.
Dave