Jamieson, Fausset and Brown on 2 Thessalonians 3:17

17. The Epistle was written by an amanuensis (perhaps Silas or Timothy), and only the closing salutation written by Paul's "own hand" (compare Ro 16:22; 1Co 16:21; Col 4:18). Wherever Paul does not subjoin this autograph salutation, we may presume he wrote the whole Epistle himself (Ga 6:11).

which--which autograph salutation.

the token--to distinguish genuine Epistles from spurious ones put forth in my name (2Th 2:2).

in every epistle--Some think he signed his name to every Epistle with his own hand; but as there is no trace of this in any manuscripts of all the Epistles, it is more likely that he alludes to his writing with his own hand in closing every Epistle, even in those Epistles (Romans, Second Corinthians, Ephesians, Philippians, First Thessalonians) wherein he does not specify his having done so.

so I write--so I sign my name: this is a specimen of my handwriting, by which to distinguish my geniune letters from forgeries.