It looks like the inspiration for designs of Darth Vader and C3PO have been uncovered. Star Wars costume designers John Mollo and Bermans Nathans may have taken inspiration from old smoke helmets. You really can't deny the likeness of the helmets pictured above.
Here is an explanation of the masks from Collector's Weekly:
"The black leather helmet on the left is labeled "Respirations Apparat" by "G.B.Konic Altona," was made in Hamburg, Germany, and has the look of an African Dan mask. The brass, three-quarter face mask to its right was made in Paris by J. Mandet.
This type of breathing mask had a very simple apparatus, allowing only a short range of operation. When used, air would be forced into the helmet through no more than 13 meters of flexible tubing by means of a bellows operated remotely from the outside. Both of these masks have mica lenses to help protect the eyes from heat."
These helmet collectors speculate that Lucas himself knew of the masks. Of course Lucas knew about them! The guy was an artist who did a ton of research in building up the world of Star Wars. It's also interesting to note that one of the 19th-century mask manufacturer was named Vajen-Bader, which sounds like Darth Vader.
It's no surprise to me that the Star Wars concept art came from designs like these. After all, artists and designers look for stuff like this to inspire them, and help guide them in the direction they want to take their vision.