proper designation of filter for old CT* Hasselblad 40mm lens

Eventually I want to buy a UV or haze filter for my older version Distagon 40 lens. Its front element is perfect and I'd like to keep it that way. There are no ordinary filter threads on this lens. Instead there is an outer bayonet mount. I've seen these filters called 104mm. They are not cheap and I want to be sure I'm getting the correct filter. What exactly is it called? Thanks.

Does anyone have a spare for sale?

BAY 104 is how Hasselblad calls them. Kinda hard to find. They do pop up on eBay every now and then.

Thank you!