Similarly as when citing a manuscript, you may be limited by the amount of information available here as well. The example below cites a letter again from an inheritance of a deceased author, the following comments will explain why each part of the bibliographic reference is written in its respective way.
Allen Garland E. (Department of Biology, Washington Univer- sity, St. Louis, MO). Letter to: [Vladislav] Kruta [(Univerzita J. E. Purkyně, Brno, Czechia). 1972]. 1 leaf. Located at: Inheritance after Vladislav Kruta, University Campus Library, Masaryk University, Brno (Czechia).
In this case almost all details could be copied from the letter except for the first name of the addressee, his affiliation and date of the letter’s writing. The surname Kruta is in the letter apparent as an addressee. Due to the fact that the letter is stored as part of the legacy of Prof. Vladislav Kruta, and his long-term work for the Faculty of Medicine of the then University of J. E. Purkyně in Brno, we could without any doubt identify the first name of the addressee and his affiliation. Nevertheless, we still strictly observe the rule of using square brackets to denote information drawn from other sources than the cited document.The date of the writing of the letter is only specified as a year, because it follows from Prof. Kruta’s notes that the letter was inserted into a parcel with a book. Considering the fact that even today delivering a parcel from the USA may take several weeks, we could only speculate whether the letter was written in May or in some preceding months.