Cal. Pub. Util. Code § 50256

Whenever outstanding bonds are refunded they shall be surrendered to the treasurer of the district, 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.