Sumitomo Metals in Japan used acronyms SDCH, SDCR and SDCO as stencils applied to aluminum used to manufacture aircraft. These black stenciled letters were applied to interior surfaces of aluminum on Imperial Japanese Navy (IJN) and Japanese Army Air Force (JAAF) aircraft.
SDCH - Super Duraluminum Clad, "H" means water-cooled after tempering at 500° degrees Celcius, cured at
a normal temperature, then bent into shape.
SDCR - Super Duraluminum Clad, "R" means rolled after being tempered.
SDCO - Super Duraluminum Clad, "O" means
annealing at 360 degrees Celcius for easier shaping, and then tempered.
References
Note, these acronyms are not any reference to any United States company or indicate the aluminum was scrap metal sold to Japan.
Japanese Aircraft
Interiors page 60
"The Japanese company of Sumitomo Metals used this
romanji script to identify the type of aluminum and its annealing state. SD stands for Super Duraluminum as opposed to ESD (Extra
Super Duraluminum), both produced by Sumitomo. C means that it is clad
with pure aluminum on both surfaces for corrosion resistance. H means the sheet is water-cooled after tempering at 500° C, cured at
a normal temperature, then bent into shape.
