SnapOn makes 4 sizes of swivel ball extension sockets (skinny 3/8 drive extension with a 10mm to 14mm swivel socket on the end). The ball portion is part of the end of the extension with a recessed groove for full motion of the socket.
The swivel portion below the ball seems like a standard SnapOn ball swivel socket, so if it isn't too hard to take the ball swivel apart it seems like it should be possible to put the extension bar on any size ball swivel socket you wanted to. If it worked well I would look into cutting a ball end on a normal extension instead of paying $20 or so for a used Snapon FLXM813.
The swivel portion below the ball seems like a standard SnapOn ball swivel socket, so if it isn't too hard to take the ball swivel apart it seems like it should be possible to put the extension bar on any size ball swivel socket you wanted to. If it worked well I would look into cutting a ball end on a normal extension instead of paying $20 or so for a used Snapon FLXM813.