When reading the spec sheet for my m250r2, I found an option for a charcoal filter. I'm assuming this is a mistake, but if it's not, what is a charcoal filter? 

Just quessing, a pressure equalization air filter of some sort, so that the air in the lantern can contract (when it is switched off and starts to cool down) without pulling in dust from outside

Any air filter will do a job of keeping dust in/out. But a charcoal filter implies noxious fumes that need to be chemically trapped. Mercury fumes?

Mercury fumes from outside (in a quantity that won't kill everyone around first) won't do anything to the light

Mercury fumes from inside (in case of a lamp explosion) are limited to the quantity of mercury that is present in a lamp, and that is not dangerous quantity in the free air. Besides, somebody will have to open the door to replace the lamp anyway

The filter allows to equalize the pressure and at the same time prevents ingestion of dust from the outside. The problem with the dust in the cities is, many of it is in the form of hydrocarbons (mainly from the car engines, but a hot soot tend to form them with the water vapors as well) and these tend to react on metal surfaces and form insulating polymer layers, compromising the functionality of contacts or so. That filter is very potent to trap these hydrocarbons. These reactions are the reason, why initially popular platinum contacts (for hi-rel relays for safety systems or telecom or so; in the clean environment of the labs they appeared extremely robust and reliable) were very soon replaced by other materials - the platinum is really extremely potent catalyst for the polymerization, so suffered the most.
And the charcoal is one of the most effective filter to trap these unsaturated hydrocarbons.

And for the mercury escaping the lantern: It would make sense in case the lamp explodes: When the lantern is operating, it is hot. That means the mercury remains at a rather high gas pressure, so high concentration and the filter prevents it from escaping the lantern.
If you are going to service the lamp, it would be already cooled down, so the mercury would condense into a liquid form (or more likely amalgam with the aluminum lantern body), so become trapped with the fixture and so won't release into the air that much.

  There really was nothing esoteric about the use of a charcoal filter in the optical system of a light fixture. GE advertised this as a major innovation in lighting fixtures in the late 1960's. The m-400a "power door" street light fixtures offered this as a standard feature. It was just something that kept the inside of the fixtures clean thus reducing maintenance costs. By tightly sealing the optical system and having the air exhaust and enter the fixture through the filter,the entrance of contaminants was minimised. I remember seeing a crew cleaning streetlights. (this was when I was walking to school in 1962). The fixtures were gumball incandescents equipped with 870 watt,15,000 lumen lamps. Each time the electrician opened a fixture , a cloud of dead insects fell out. All he did was wipe the reflector,globe and bulb with a wet rag. About ten years later,I watched a crew cleaning fixtures on a parkway. The driver just glanced at the light to see if it looked dirty. They did not stop to clean any of the m-400a fixtures. The filter kept them clean.

I have a few GE floodlight fixtures that have the charcoal filters-they do work! One fixture is 400W HPS-other 400W MH.The filters do well in keeping the lenses,reflectors,and even the bulbs clean!

I have a few GE floodlight fixtures that have the charcoal filters-they do work! One fixture is 400W HPS-other 400W MH.The filters do well in keeping the lenses,reflectors,and even the bulbs clean!

Can you post a picture? Also, do they have fans on them? I really want to see what they look like. 

  EBay item # 1517 6374 8789 is a 1969 ad from General Electric explaining how the charcoal filter works in the m-400a street light luminaire.

http://www.lighting-gallery.net/gallery/displayimage.php?pos=-57168

Not charcoal in this one, just cheap rolled up plastic wool. (And it is pretty useless in this lantern, since the opening around the perimeter of the cover is not sealed anyway)

Those lights are still packed from a move.The filter device is a button like device that has the charcoal in it behind a small metal screen.It is on the bottom of the fixtures to protect the filter from rain or snow.No fans-the filter works by the expansion of air as it heats up in the light while it runs-then goes out of the fixture when it turns off and cooled.It is maybe like half in or so in diameter.