Any person electing to secure credit for prior service as a forester, firewarden, or fireman under this chapter who has to his credit in any other pension system a sum less than that necessary to entitle him to credit for such prior service, shall deposit in the retirement fund a sum equal to the amount necessary to entitle him to the credit claimed for prior service, and the amount he has to his credit in the other pension system shall be transferred from such other system to the retirement fund of this system, to his credit. The amount necessary for such credit shall be determined on a 2 percent basis applied to the aggregate salary or wages paid during the period of prior

service claimed.