Our counter guy had to come back to grind the granite to enlarge the opening for the Bosch unit to fit. Took him a minute or two.

" Is the 19 7/8" absolutely necessary, and why? "

Something tells me if that is what they say, it is because that is the size opening needed for their cooktop to fit properly with the necessary clearances..

Asking because I found a second-hand Bosch 800 unit at a very reasonable price; the seller says the actual chassis is only 19 inches, so wondering what doesn't fit.

This is a rental with a 14-50 plug under the countertop unit and I'm looking to swap it out for induction while I'm here. The opening is 35 x 19 1/8.

I ahve no idea but when it comes to this type of install better safe then sorry I have no idea what a 14-50 plug is but you need mega electrical for induction so make sure about that .

So go look at the cook top bring a measuring tape and the install directions with you .