You can search according to the following conditions:

Sequence number: For example: s0013
Date of letter:For example: 1992 or 1992-12 or 1992-12-12
Address of author: For example: Jiangsu Province or Nanjing City, Jiangsu Province
Date of event: For example: 1936 or 1936-12 or 1936-12-13
Location of event: For example: Nanjing
Name of author: For example: Zhang Er
Name(s) of victim(s): For example: Zhang Er’s father