Some boats are measured LoA. Some boats are measured LWL. Some boats include the swim platform, some do not. Some models used one method, then at a certain model year, switch to another. I've never seen a boat who's LOA matched the badging.
My 350 model, was named the 390 just one year later. I have two friends with identical Carver aft cabins, but two different model years. One is a 36, then other a 370. I have a friend with a Silverton 392, that in later model years was badged the 372.
Unless you're trying to dock beam-to between two boats (parallel parking style) it doesn't really matter.
Incidentally, go by the badging on teh boat. If it's a 1900, say it's 19ft. If you say 21 ft, it looks a little petty.
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