Registered cover franked with 1919 5c on 50c grey 'Provisional' SURCHARGE issue with TRIAL OVERPRINT IN YELLOW and 6c on 25c brown with TRIAL OVERPRINT IN RED all tied by 'Lines' cancels with NEGOCIADO DE REGISTRADOS DEL INTERIOR cds dated FEB 23 1921 with boxed 'SAN SALVADOR' registration marking alongside both in purple. Addressed to SAN MIGUEL. Fine and rare and most probably a unique franking utilising the trial overprints.