Think that is just a "Push-fit" tail, the little recess is so the snap ring has something to grip?
Correction they're not called a tail they are called a stem !!!
https://www.hydraulicmegastore.com/product/6811-8-1-8/
You may just want to change the fitting at the body of what ever it is, to the tail you want and bin these, as they are a PITA, the push fits sometimes won't let go!!
Think that is just a "Push-fit" tail, the little recess is so the snap ring has something to grip?
Correction they're not called a tail they are called a stem !!!
https://www.hydraulicmegastore.com/product/6811-8-1-8/
You may just want to change the fitting at the body of what ever it is, to the tail you want and bin these, as they are a PITA, the push fits sometimes won't let go!!