Section 21. A document made or received by the Massachusetts Bay Transportation Authority Retirement Board or any other legal entity, public or private, which receives funds from the Massachusetts Bay Transportation Authority for the payment or administration of pensions for employees of the Massachusetts Bay Transportation Authority shall be considered a public record under this chapter and under clause twenty-sixth of section 7 of chapter 4 and subject to all applicable exemptions; provided, however, that subsection (6) of section 23 of chapter 32 shall also apply to these documents.