Under "Understanding valuing notifications" in the Introduction section of the Scott Catalog, this is addressed. It says a dash in the value column means that the stamp is known in a stated form or variety, but information is either lacking or insufficient for purposes of establishing a usable catalog value.
My old 2009 SG West Africa catalogue lists two stamps like this from June 1999:
SG 741a Perf 14
SG 741aa Perf 13
with both priced at £2.00 mint and 60p used.
re: Nigeria #694A
Under "Understanding valuing notifications" in the Introduction section of the Scott Catalog, this is addressed. It says a dash in the value column means that the stamp is known in a stated form or variety, but information is either lacking or insufficient for purposes of establishing a usable catalog value.
re: Nigeria #694A
My old 2009 SG West Africa catalogue lists two stamps like this from June 1999:
SG 741a Perf 14
SG 741aa Perf 13
with both priced at £2.00 mint and 60p used.