M240 doesn't have a nice device, unlike M-P (240) which has the field lines lever.
Just try this before sending for repair :
1 - Mount your Nokton 50mm lens, as usual on the M240
2 - Look in the VF, you may see the "50/75" field
3 - Carefully depress the lens release button, and turn the lens clockwise (looking from front), you may hear a "clic" or not when field lines change
4 - Carefully look in the VF holding the lens and the M, not to let the lens fall off
5 - You must see the "35/135" field lines
6 - If not any change, the field lines selection is faulty.