I did exactly this a while back using cheap Chinese calipers. During my research into the protocol, I came across the Mitutoyo information, and also having a Mitutoyo caliper, I was interested in decoding it too. However their serial protocol is much different, so I opted to skip.
Because you already have this "input tool", I would focus on using it. Let the tool take care of the manufacturer-specific protocol between the caliper and the tool, and just focus on the RS232 interface side. This will probably save a lot of coding time. I'm sure there is a micro inside the tool running code that does the translation - no need to reinvent the wheel.
As for interfacing the "input tool" to the Pi, you'll need to convert the RS232 interface voltages down to something that the Pi can handle.