The Smithsonian's National Postal Museum's ongoing four-part online exhibition of "Women on stamps" continues with parts three and four.

As the title suggests, the expo features important women who have literally left their "stamp" on history, beginning with the first image of a female on a stamp in 1893.

Part Three will focus on women who have made significant contributions to literature and the arts; including journalists such as Ida Tarbell and novelists including Edith Wharton.

The Wizard of Oz stamp, issued on February 18, 1990

Afterwards, Part Four will centre on women from the performing arts, from pop gospel singers to stars of theatre and dance - with various prominent figures from each.

The Audrey Hepburn stamp, issued on June 11, 2003

Famous faces among the philatelic subjects include Wizard of Oz star Judy Garland, Marilyn Monroe and Audrey Hepburn.

The National Postal Museum's expo coincides with Women's History Month, an annual declared month in the US highlighting the contributions of women to events in history.

Elsewhere, in May of last year, a rare stamp featuring Audrey Hepburn sold for $93,800.

Hepburn's son had previously ordered that the run of stamps be destroyed, objecting to its depiction of his mother with a cigarette.