Citing primary sources can be challenging. Unlike other publications, primary sources might have complicated publication information and will not fit the standard format for the citation styles you might be used to. Additionally, primary sources come in various formats and/or may be unpublished and only accessible virtually in a digital archive or in a physical archive.
When in doubt as to proper citation, it is best to remain consistent by citing the same types of materials in the same fashion using your best judgment.
Citing a primary source document, from an archives, varies depending on the preference of your instructor, the publication you are submitting the article, or the discipline in which you are operating...
There are several elements to the citation which may be necessary to properly cite a document such as the Booth Tarkington letter to the left.
The document: The creator or author, specific item title, page, section, date.
Box and file number: The majority of materials found in archives, that are not books, are housed in some sort of container and within the container the various items are separated for various reasons in files.