Your serial number will be located inside the soundhole, up towards the headstock. Please note that serial numbers for the following models are not logged in our system: Protege by Córdoba, UP100, GP100, CP100, CP110, C3M, C3M Cadete, Requinto Cadete, Dolce, C5, C5-CE, C5-CET, C5-CEBK, C5-CETBK, La Playa Travel, La Playa Travel SS, La Playa Ukulele, Minis, and all ukuleles except for the 25 series and 30 series.