[XJarhead]

Uh, wait a minute. If you're sending it as the salutation on a letter, its just "LtCol. Smith". Don't add "USMC (ret.)". The latter is appropriate if you are referring to his title in a writing other than a salutation.

[Khurkris]

Probably so. What I posted is what a buddy who is a retired LtC from the Marines uses when he writes complaint letters -(he forwards copies to me).
I made SSGT. 3 x's and never could figger out if that made me an E-18...seemed logical to me.