I have in my register a Swedish 4-masted schooner with 391 grt, 267 nrt and 550 tdw. John A Campbell seems to be much bigger with 546 grt. Otherwise I was thinking if it could be 650 tdw. But in my opinion she should be about 800-900 tdw. For a sailer they used to say tdw is about double of nrt. I will see if I can find her dimensions."Gross tonnage" refers to total enclosed space (1 'ton' = 100cu ft) Underdeck tonnage likewise but below 'tonnage deck' - generally uppermost continuous deck. Cargo carrying capacity measured either by weight (deadweight - weight of cargo stores fuel, etc necessary to bring ship down to plimsoll mark) or volume eg "Measurement tonnage" : 1 ton = 40 cu ft of cargo space. I suppose M could refer to 'Measurement' but for a sailing ship with 462 under deck tons (46,200 cu ft) 650 Measurement Tons (= 26,000 cu ft) seems too little.