I'm not aquainted with dr. Wright, but I'm well aquainted with the Vade Mecum!

Back when plates were silvered copper plates, the sizes were standardised on the sizes of the best locally available plates made for the printing industry. So full plate was different in France, Germany, UK, USA and everywhere else. This carried over to the glass plates, as the cameras already existed. Some "merging" of standards occurred over the years, and is still occurring...
Two early Norwegian photographers, Knut Knudsen and Anders Beer Wilse, are easily distinguishable by the size of plate they used: Wilse was originally Swedish, and used German plate sizes. Knutsen, from Bergen, used English plates!

Come to think of it, I think I have some half-plate holders in 5x7" (holder) size somewhere. A faulty description on ebay, with a price so low that it would cost me more to return them. I haven't used them, since I find 13x18 and 5x7" to be confusing enough without using a third size in the mixture.