The Halkey-Roberts valve body (which you see on the outside) is one piece. Inside the tube, there is a threaded collar that spins onto the body, trapping the fabric between the two flared portions, which makes the seal.
To remove the valve, you need to unscrew the inner collar (or if you look at it the other way, unscrew the body from the inner collar.)
The first pic shows the complete valve without the tube in the way. The second pic shows one style of valve wrench available.