deskthority

On my IBM Model M, when I press SHIFT+Left in a text editor, the letter at the left of my cursor is highlighted.

On my IBM Model F XT and IBM Model F AT, when I press SHIFT+Left in a text editor, the character 4 is emitted because the Left key is on the numpad, shared with the character 4.

Is there a simple configuration using the Soarer converter firmware that I could use to have the same behavior as my IBM Model M (or any modern keyboard...) ?

You might be able to use a macro like:
You press the Left Shift key with number pad 4 to activate this macro. You can press Right Shift with number pad 4 to get the regular behavior. You might have to replace, "PAD_4", with whatever string corresponds to the key code that your keyboard emits when you press Left Shift Left Arrow. You can run the, "hid_listen.exe", program to find out what key code that is.