Head: Indiana Jones- Russian Soldier
Neck: Ink pen body
Helmet: 25A Iron Grenadiers
Torso: ROC Storm Shadow
Arms, gunbelt: ROC Street Viper
Legs: NS Viper
Sidearm: ROC Snake-Eyes (arctic assault)

Design:

The Sergeant Major first appeared in Marvel Comics, though I can't remember if the "Duncan" name was ever attached or not until the GI Joe Collectors' Club rolled out a nice version of the character in 2011. The GIJCC version used a Sgt. Stone head, which seemed a bit too elegant for how I remembered the character in Marvel Comics.

This custom has the Sergeant Major decked out in a field-ready Iron Grenadier uniform. Instead of slapping an unmasked head on a 25A Iron Grenadier body, this custom gives SM his own personalized uniform. It also features a head that's a callback to SM's first appearance in Marvel's GI Joe #57, in which the Iron Grenadiers (not yet given that name) wear garrison caps and kilts. It's a mild hybrid of that first uniform, seen in GI Joe #57 and the more iconic black uniforms of the '88 action figure first shown on SM in GI Joe #69.

The Indiana Jones Russian Soldier head provides a garrison cap. It also has a gruffer face that is a better match for SM's first close up images in GI Joe #69. The ROC Storm Shadow torso is a strangely adaptable one, here working as good stand-in for an Iron Grenadier uniform.

Colors & Paint:

Black, red, and gold. He's an Iron Grenadier. Plus light grey hair to match the Marvel Comics appearances. The garrison cap was plaid green/black in the Marvel appearance, but I switched it to black/red to better fit the replacement IG color set.

Sculpting & Modifying:

The neck is the body of a pen tube, glued to the head at one end. At the other end is a small o-ring to create a collar.

Thanks for looking.