In a 1951 letter, the Rebbe returns a reviewed letter, notes a misprinted version of the Alter Rebbe’s release letter, and encloses Chassidic texts. He inquires about missing maamarim of the Rebbe Rashab, asking if the recipient recalls any not listed in the index.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/shevat/890