California Code, Public Utilities Code - PUC § 101321

Whenever outstanding bonds are refunded, they shall be surrendered to the treasurer of the district, or the county treasurer, as the case may be, who shall cancel them by endorsing on their face the manner in which the refunding was effected (whether by exchange or purchase, and the amount for which so purchased, if any) and by perforating through each bond and each coupon attached thereto the word “canceled” together with the date of cancellation.