Every overflow pipe I've seen has a hole in the top similar to yours. I've not seen a clip like is on yours. I don't have a good pic that shows the hole. Somewhere in my "archive" of spare parts I have an NOS piece that I'll try to find and take a picture of.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.