Below is a citation (p. 129, under Plusia nichollae) from: Lafontaine, J. D. and R. W. Poole, 1991. Noctuoidea, Noctuidae (part) in Dominick, R. B. et al., The Moths of America North of Mexico, fasc. 25.1
"Plusia nichollae is very similar to putnami, and in the West the two species are frequently confused. Most specimens of nichollae can be recognized by the more oval basal spot in the stigma; this spot extends dorsally to the cubital vein or may be represented above the vein by a small silver speck, whereas in putnami the basal spot of the stigma extends above the cubital vein in most specimens, giving it a more triangular appearance."

In this picture Cu is the cubital vein, which is colored by blue. BSS is the basal stigmal spot and DSS is the distal stigmal spot. ACE of BSS is the antecubital extension of the basal stigmal spot. This specimen could be confused with Plusia putnami because of the extension of the basal stigmal spot. In P. putnami the extension should be large giving to the whole spot more triangular appearance. To compare pictures of both species of Plusia see the reference above. However, these two species of Plusia are very close and problematical specimens should be dissected and genitalia should be checked for correct ID.