OK, I found the problem. It was that every character in the node class name must be lower case (minuscule pour nos amis francais) except the initial 'N'. I had created class Node_boxCLT in the file node_boxCLT.py and with icon file node_boxCLT.png. That didn't work. Class Node_box_clt in file node_box_clt.py with icon file node_box_clt.png does work. In the menu that offers to create a node, this shows as box_clt.