Hello,
I was searching for long for such a plugin. So, first of all thanks for it, it's quite useful to select all the contour lines at one level (topography).
I just wondered if it is possible to make a small improvement on the execution procedure.
In addition of the current method (select one edge or face and then execute the plugin), could it be possible to execute the plugin first and then enter a Z value on the VCB (I don't know if the name is correct, the little box on the right bottom 
) and finally validate with "enter"?
In order to keep the current method, the general procedure could be:
- Execute the plugin
- Enter a value on the VCB OR select an edge or a face
- Push "enter"
At each step some short information could be displayed on the status bar.
I just discover ruby and its power and unfortunately I don't really have the time to learn it so that I can make this modification.
Thank you