Until a member pipes in with an ID - I searched Google and found that letters, etc. are used to help identify nails - didn't know that! Couldn't find "ST" but you have many links to check out. Here is just one of many links...
[FONT="]osage express[/FONT][FONT="] nailed it. (Pun unavoidable!) [/FONT]
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[FONT="]"ST" is attributed as "Steam Treatment [or] Special Treatment. Santa Fe" and also as "Spiral to Tangent. Baltimore & Ohio" in Date Nails and Railroad Tie Preservation by Jeff Oaks. See listings on p. 80 and p. 85 at