The Cammenga U.S. Military Lensatic Compass.  MIL-PRF-10436N. 

Here is a brief overview of the unique features and history of this compass. 

The US Military Lensatic is one of the most rugged and reliable compasses in the world.  There are 3 main features that set the Cammenga Compass apart from other compasses on the market.  The most noticeable feature is the use of Tritium Light Sources integrated with the dial, bezel and sighting wire.  Tritium light sources are sealed glass tubes that are filled with tritium (3H) gas and have their interior surface coated with phosphorous paint.  The tritium causes the phosphorescent material to visibly glow for about 20 years, however, the glow gradually diminishes after 12 years wich is the half-life of tritium.  Tritium light sources are aslo used in aircraft gauges, gun sights, emergency signs and a variety of other critical devices.  When buying a compass remember that after 12 years, all tritium light sources gradually fade.

Cammenga model 3H at night----->     

Another Unique feature of Cammenga's compasses is the use of electrolytic induction damping to steady the rotation of the magnet.  Other compasses steady the magnet by filling the compass with oil wich can leak, form bubbles and eventually make the compass unusable.  Induction damping works by surrounding the compass magnet with copper.  This is painted white and is what you see around the dial of U.S. Military compasses.  The copper causes neutral magnetic resistance to the magnet as it pivots thereby slowing the momentum of the magnet.  If it weren't for induction damping a compass magnet would wobble around magnetic north for almost a minute before coming to rest.  Cammenga compasses lock onto magnetic north in under 6 seconds.

Finally, Cammenga's compasses are built using an aluminum casing.  This makes the compass extremely rigid and durable.  The waterproof magnet capsule and tritium vials remain protected throughout the life of the compass thanks to it's aluminum frame.  The compass can handle extreme environments from -50ºF to +150ºF and is shock, water and sand proof. 

If you have a Cammenga compass or are looking to buy one here is an overview of what to look for.  This covers some of the history of the compass and the different models produced by Cammenga. 

The U.S. Military Lensatic Compass was designed in the 1950's.  A few different companies have been awarded the DoD contract to manufacture the compass over the years.  Cammenga was awarded the contract in 1992 and is the only manufacturer of the U.S. Military compass today.  This exclusive contract with the Department of Defense makes Cammenga the World's largest manufacturer of military compasses. 

Original mil-spec compasses from Cammenga, 1992 to 1999, have two distinct features,  1. The bezel is made of aluminum.  Aluminum bezels have a slot located on the side ridges  2. The markings on the back of the compass read:

Pre 1999 U.S. Military---->   

Mil-spec compasses made from 1999 to Present, 1.  Have a plastic bezel (No slot).  2.  The markings on the back of the compass was changed to read:

After 1999 U.S. Military---->   

In addition to the U.S. Military, Cammenga sells the tritium lensatic compass commercially (to civilians) .  The commercial compass is exactly the same as the military compass except for the markings on the back of the casing. 

Commercial 3H---->    

There are some variations of the examples shown above.  If a compass has the numbers 21-26460-02E / 120mCi 3H and the Cammenga logo engraved on the back cover (the back cover is the part that is attached to the case with screws) it could be either a commercial or U.S. Military compass from pre 1999.

There are also 2 additional models that Cammenga produces based on the mil-spec compass design.

The model B3H is a "S.W.A.T." black version of the mil-spec compass.  From what I know this is not used by the U.S. Military but is sold to law enforcement agencies and commercially. 

The most popular is the model 27 wich uses phosphorescent paint instead of tritium for its light sources.  All other specifications of the model 27 compass are identical to the Tritium models.  These compasses are sold commercially and are not used in the military.  They cost about 1/2 the price of the tritium models and are a good economical alternative, but they loose their glow after about 2 hours in the dark and must be recharged with light in order to glow.  The backs of the model 27 compasses look like this:

  

Also, In 2002 Cammenga began packaging all the compasses with protective tape covering 1/2 of the magnifying lens.  This tape simply protects the lens from getting scratched during shipping.  If you see a compass without the protective tape it means the compass was either made before 2002 or is newer but has been used or had the tape removed.

The two rarest versions of the Cammenga Compass are easily distinguishable by their unique colors.  

Around 3,000 RED compasses have been made for the U.S. Military.  On the RED models the magnet is reversed on the dial so that the North of the compass dial actually points South!  It is uncertain what these compasses are used for.  A likely theory is that they make it possible to navigate towards someone reading from a regular compass without converting the numbers.

Cammenga also produced 300 BLUE compasses for a special order sometime around 1998.

The information in this Guide came from Cammenga's web site, Cammenga's e-mails in response to my questions, and my own personal experience using the Cammenga Compass.