(1) The maximum rate at which an operating agency shall add interest in repaying a member under RCW
43.52.391 may not exceed the higher of fifteen percent per annum or four percentage points above the equivalent coupon issue yield (as published by the Board of Governors of the Federal Reserve System) of the average bill rate for twenty-six week treasury bills as determined at the first bill market auction conducted during the preceding calendar month.
(2) The maximum rate specified in subsection (1) of this section is applicable to all advances and contributions made by each member to the agency prior to January 21, 1982, and to all renewals of such advances and contributions.