I selected a couple of tinclads to be built in St Louis. The next turn I noticed that they are listed as cottonclads. In particular, the force is named "cottonclad #1" and the unit says "cottonclad" across the bottom, but the two elements are tinclads and when I click on them they appear to have tinclad stats.
So I'm guessing the database just has the name of the unit wrong.