Let Steve correct any mistakes here, but IINM, from looking at the ZS specs, the circuitry is the same, and the major difference from the earlier Select is in the chassis.

Personally, while there was an undeniable charm to the original grey and black metal boxes, the updated format with solid hardwood base and thicker machined aluminum top plate with a larger footprint is a big improvement, and coincidentally (or not  ??? ) just might provide more room under the hood for DIY tweakers, or future design upgrades.