Well, "Gun #22" has got one! #24 and #25, though perhaps a bit later (1780's-1790's???) have boxes that are not too dissimilar to that on the presumably-earlier Gun #22. I would have no problem whatsoever considering the metal box that is on 22 as being Revolutionary War period.
My personal opinion is that the series of 20-22 (and 26) are probably from about the 1770 mark... give or take. Some of the others appear later, but perhaps not by a whole lot.
I got to see and handle the famous #21 some years ago. It is awesome. There are shaping details that I would NEVER have gotten just by looking at the picture books. Berks guns can be almost as unique (and difficult!) as Lehigh guns.