I also have these models and tried to understand how to put them together and I gave up. After much searching I'm tempted to configure the sails according to this: https://www.stefanoni-modellismo.it/...roller=product

Like the Langton Langton K5 model, this example has one of the masts in the stern instead in the bow. Most of the other pictures of gunboats had a mast in the bow.

However the Langton model has the mast at the very back and the example in the image above is not quite so far back, so I'm not sure...

Of course, one could modify the model according to any of the appropriate examples found by searching for images of "18th century gunboat"...