What are the different kinds of blowstick valves?

There are a number of different approaches to the problem of keeping air in the bag while the piper takes the next breath.

Tongue: The original valve was probably the tongue.  However, this is a clumsy and inelegant approach.

Leather Flapper Valve: Most pipers are familiar with the leather flapper valve.  The issues with this valve are that unless you play regularly, the flappers will leak.  They can be oiled to keep them flexible.  Because the "hinge" is a weak point in the valve, the blowstick should be arranged such that the flapper hangs in a position such that gravity assists in its closure.

Merrick Valve: I don't know much about this valve except that it was a spring loaded ball valve in the upper part of the blowstick.  My understanding is that it worked quite well.

Little Mac Valve:  This is a rubber sheet check valve which works on the same principle as the Leather flapper, but tends to be more reliable.  My issue with this valve is NOT in its ability to seal, but in the fact that the bottom of the blowstick must be reamed to accept the device.  This weakens the blowstick and, when you wedge the Little Mac into place to get a good seal, cracks often result.  Because the blowstick has been reamed, going back to a traditional type flapper is not possible as the hole is too large to seal well. (See How do I fix my blowstick?)  In addition, the inner diameter of the housing is narrow and adds length (i.e., more restriction) so blowing is actually harder than with a simple flapper valve.

Artificial Flapper Valve: This is a duplicate of the leather flapper valve using a brass stem and a rubber gasket material for the actual valve.  In my mind, this is the best of the leather flapper and the Little Mac all rolled into one.   Because of the natural "spring" of the rubber gasket material, there is no need to be concerned about orientation.  Vertical adjustment can be a bit touchy to avoid gaps near the brass stem, but this is readily adjusted.  I routinely use this valve when relining a split blowstick or repairing the damage caused by a Little Mac.  This valve works great in smallpipes too.   Naill makes this bit for about $3 at most shops.  Gibson uses it on their blowstick.

AirStream Blowstick:  This is an all in one solution to several blowstick problems.  The valve is internal to the blowstick and is extremely reliable. It is that it is possible to accidently overtighten the sealing surface of the internal valve by twisting the blowstick in the stock during routine use.  This may pucker the valve sheet causing leaks.  This can usually be remedied by backing off on the internal threads.    Valve failures are rare, but replacement flappers are available for $2 at most shops.  The oval shaped mouthpiece is really nice and, when covered with a bit of latex tubing, is very gentle on your teeth.  Your dentist will appreciate you for springing for this!

Gibson Telescoping Blowstick:  This uses the Artificial Flapper Valve.  (Note: This blowstick has a slightly larger internal diameter.  The telescoping blowstick is a rather nice way for an individual to find what length of blowstick to use.  IMHO, every band should have one just for that purpose.)