You lost me there for a minute. Underslung has a couple definitions and I'm using this one.
"suspended from the underside of something"
I'm not referring to this one.
"The axle position in relation to the spring needs to be considered before building. The options are “overslung”, where the spring is mounted over the axle, or “underslung” where the spring is mounted under the axle."
Yours, mine and the ad are underslung trailers.
The only similarity I see is that it is an underslung trailer. It does fit the time period as I think the boat roadster trailers were made from '46-'53 and were refined through the years. There were other companies that made underslung trailers and I believe some homemade copies. I have one, see my avatar.
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1950 Skipper on Holsclaw Boat Roadster trailer
1954 Skipper w/ ’54 Johnson 10hp
1954 Deluxe Runabout w/ ’54 Johnson 25hp
1954 Vagabond w/ '57 Johnson 35hp
1955 Alumacraft FD on a Gator trailer
1956 Alumacraft FD
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