"The charcoal iron is a self-heating device similar to a miniature cooking stove. Charcoal from wood or wood coals was normally used for fuel. Heating control was provided by vents which controlled the air/oxygen supply." David Irons, Irons By Irons, page 33.

An easy way to tell the difference between a charcoal iron versus a slug/box iron is by the presence or absence of air venting. Burning charcoal needs air to fuel the fire, so vents (or a chimney) are necessary in a charcoal iron. A slug iron outer casing is solid to retain the heat of the hot metal slug.

R. Kuhfs ~ Patent Model Charcoal Iron

Patent Model Charcoal Iron by R. KUHFS, ST. LOUIS, MO., PATENT AUGUST 11, 1863/ Patent number 39,484. Comes with original tags and paper. Sold by Hartzell's Auction Gallery at the Balestri Auction on 7/12/18 for $525.

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This YouTube video demonstration reminds us that the charcoal iron is still used today in many undeveloped areas of the world. 

Charcoal Irons are still being manufactured today. Here is an ad showing one particular model, the  "Cock Brand", produced and distributed by Linyi Hantensi Trading Co., Ltd. of China. According to their website the main markets for these irons are Africa, Haiti and South America.