40 ILCS 5/19-213
Every person who is in the employ of the board of directors of such library when "The 1905 Act" goes into effect and who intends to become a beneficiary of the pension fund created thereby shall, on or before the 15th day of November succeeding the election of said board of trustees, file a statement of such intent with said board upon blanks prepared for that purpose. Every person who becomes an employe, after "The 1905 Act" has taken effect and who intends to become a beneficiary under "The 1905 Act" or this Division shall within 6 months after such entry file a statement of such intent with said board of trustees upon blanks prepared for that purpose.
Provided that after July 16, 1941, employes of the board of trustees of such Public Library Employes' Pension Fund, who become eligible to participate in the benefits of said fund shall have the right, upon filing a statement of intent with the board of trustees, to become a beneficiary of such pension fund, to contribute to such pension fund an amount equivalent to the contributions which would have been paid to that date had the person become a contributor at the time "The 1905 Act" became effective or became a contributor at the date of his entry into the service of the board of trustees of the Public Library Employes' Pension Fund, and to be credited with length of service for the period for which contributions shall have been paid.
40 ILCS 5/19-213