I don´t see that many differences - a rangefinder is a rangefinder is a rangefinder.

The Blik was probably build in higher numbers and it is technically similar to the Watameter I, they both are a basic external RF.

From http://www.corff.de/Klappkameras/Klappkameras.html - Das Klappkamera-ABC, a decent guide to folding cameras:

This is my Watameter II on my Adox Golf:

This is a Blik, not mine:

And this is the german manual for the Watameter:

Differences are in the meter scale (internal (in the viewfiner, II, Super) or external (I, Blik), the ranges (the "Super" has two ranges for close and distant measurings), there even exists a Watameter with built in lightmeter.

They are helpful - but usually I guess the distance (with high hit quote) and apply hyperfocal focussing.