EX407K - It is a kiosk-based exam based on Ansible 2.3
EX407V27K - It is a kiosk-based exam as well but on Ansible 2.7
Regarding your second question.
If you want to enroll in the exam, then you need to schedule your exam in the Red Hat Exam scheduler.
If you want to unroll from the exam - I believe there is a way to manually unenroll from the exam. If you will get some trouble, you may reach the support for help.