Primary sources are the raw materials of scholarship - original sources of information that have not yet been filtered through analysis, examination or interpretation. Primary sources are often contemporary to the events and individuals being researched. Typically, when we speak of primary sources in literature we mean either the literary work under study (Mansfield's short stories) or personal information produced by the author (letters, diaries, manuscripts, and archival papers).
Scholarly and critical editions will typically present and discuss variant versions of a text; provide historical, cultural and linguistic context; and delineate the history of scholarly approaches to that text. Most scholarly editions will have an editor (or editors) and will be published by a university press.
Collins Library provides access to many journals from the modernist era and the Modernist Journals Project (linked below) can connect you to the full text of many others.