Provided that no loan or advance shall be made under this section to a State co-operative bank unless such loan or advance is fully guaranteed as to the repayment of the principal and payment of interest, by the State Government but such guarantee may be waived by the National Bank, if other security to the satisfaction of the National Bank is furnished, or if, for reasons to be recorded by it in writing, the National Bank is satisfied that the guarantee or other security is not necessary.