data relating to state assessment of private railroad companies whose cars operate in interstate commerce to tax officials of other states having duties corresponding to those described by this part. The disclosure shall be limited to instances in which there is a reciprocal exchange of information by the states in which the cars operate, and shall be made only pursuant to a written agreement between the agencies involved. The agreement shall provide that any request for information be in writing, shall specify the information to be exchanged, and shall require that any information furnished be used solely for tax administration purposes and otherwise shall be held secret. The request for information and any written material furnished pursuant to the request shall be open to inspection by the person to whom the information relates at the office of the board in Sacramento.