Have a few questions re three sets of Nigerian stamps. Nominally Scott 291 - 307. These are the stamps with the "N S P & M Co Ltd" imprint on the bottom.

For starters, is the 1975 -79 series (WM 379) litho or photogravure?

Has this group of stamps resulted in some head scratching for anyone else?

Checked the SOR archives, no discussion of this matter. Same re a couple of other philatelic discussion groups.

TIA.