This is Japanese F1 Wgayu. The cut is called Zabuton or Denver steak(in the US). This is one of getting popular cut for steaks.
Denver steaks are made from the serratus ventralis muscle, which comes from the under blade portion of the chuck roll.
The beef chuck is the shoulder of the animal and it gets a lot of exercise. That means most of the muscles in the beef chuck are pretty tough. The serratis ventralis, however, is one of the exceptions; it’s situated directly underneath the shoulder blade bone and doesn’t get used much. This makes it one of the more tender muscles in the beef chuck.