These are the new Rob Allen muzzles, open and closed AKA Low Profile or LP.
The new Rob Allen open muzzle features a deep track that helps keep the spear on the gun when wrapping the line, making loading a little easier. You can see more detailed pictures of the muzzles here NEW Rob Allen Speargun Muzzle.
The new Rob Allen closed muzzle is now called Low Profile or LP. It's similar to the open muzzle the difference being the track is closed. It is lower in profile than the old RA closed muzzle, hence the name I guess. Rob Allen Tuna spearguns can be ordered with an open or closed muzzle. But the track of the open muzzle is so deep I see little reason to opt for the closed muzzle.
Rob Allen spearguns are available from spearfishing.store Rob Allen Tuna Aluminum Railgun Speargun.