Those look really nice. Like to the point where I want to install something that needs a switch so I could use one.
However, they seem to be designed to replace the front half of a clutch lever clamp (they specifically mention hydraulic clutches). From what I can tell, the 300 has a top/bottom clamp rather than a front/back.
Random Google pic
The 500 perch that a lot of people switch to for adjustable levers has the front/back style though.
I just went out and measured mine, and it uses the 36mm spacing like the 600/636, if anyone's considering this.