Wilbur Hubbard wrote:

The photos I Googled showed both of them sailing together and neither on
had a fore and aft sail on the mainmast. Both masts were square-rigged
from top to bottom.

http://www.tallshipsyouthtrust.net/d...t=693&doc=6823

That picture shows them from ahead which makes it difficult to see
whether the gaff sails on the main masts are present.

Go to their homepage http://www.tallshipsyouthtrust.net and there
is a photo of one of them from less directly ahead, where you can
clearly see a gaff sail on the after mast (and the mainsail, i.e.
the bottom-most square sail on the main mast, is present, but furled).

On the menu along the left, click on "The Ships" (which takes you to
the page you mentioned, with the photo of the two side by side), and
then on "Our Tall Ships". This takes you to a page which contains a
photo from the quarter, which makes it a bit clearer.

On the same page there is also a sail plan with names, and the gaff
sail is identified as a "spanker".

This means they are rightly called 'brigs.'

We're agreed on that, at least.

If they were fitted with a
gaff mainsail at some time earlier or later then they should be called
'brigantines.'

Not if the gaff sails were there *as well as* the square sails.

A brigantine has no (provision to set) square sails on the after mast
at all, but if a brig (which of course has square sails on both masts)
also carries a spanker (as it usually does), that doesn't make it a
brigantine.