In Rex Stout's WWII-era novel,"Booby Trap", Major Archie Goodwin is fetching Sergeant Bruce, a fetching WAC, from her apartment to Corporal Nero Wolfe's brownstone, and in leaving, she grabs her "peck-measure cap" off the table. It's not clear whether she is in uniform or wearing what Stout calls "cits".
Is this a cap shaped like a peck basket; that is, like a fez?
A peck is almost 2.5 gallons. Is this a hat roughly 1/4 the size of a Texan's oversize cowboy hat?
 Google is generally my friend, but not today, it seems.
I was searching the term because I came across it in the exact same book. I've tried several search engines but can't find another such reference. Did you ever find an answer?
Archie tends to be pretty cutesy with his language, so when I read that book a couple of decades ago, I imagined that he was talking about this:
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It is sorta like a cowboy hat, only smaller, as befits a lady..
I'd like to reread it, but I gave away my hard copy when my eyes got bad, and paying another $8 for a book I already bought once (for $1,95) is galling. I'm tempted to check The Pirate Bay....
So, apparently, a 2-gallon hat, or 20% of the standard cowboy model.Â
I've read most of the Nero Wolfe books, but I've never heard of this one. With no more evidence to go on than what you've given, I'd guess that if Archie identified her as a WAC and her headgear as a cap, she was in uniform. Dressed as a civilian she would have worn a hat. How's that for working with nothing?
Edit: MW Table of Weights and Measures puts a peck at 8 quarts.
There are two different quarts - a peck is 8 dry-measure quarts, but 9.31 regular quarts. Does a texan use a hat sized in strawberry quarts or tomato juice quarts?
Booby Trap was a novelette, originally published in a long-defunct magazine in 1942, and then published in the back of "Not Quite Dead Enough". According to Wikipedia, there was an episode based on "Booby Trap" in the TV series, called "Gambiit", but I don't trust Wikipedia much, especially since Stout wrote a novel in 1962, he entitled "Gambit".
Not that Hollywood is to be trusted, either. Archie talks all the time about Wolfe weighing 1/7 of a ton, but he's never played by an actor weighing less than a hundred pounds more than that. Actually, I think Archie is wrong, because Wolfe's movements are more like a man weighing 400 than one weighing 280.
In the story of Gambit, Wolfe starts off destroying Webster's Third, because it's descriptive, unlike the Second edition, which is prescriptive. Wolfe clears the young ladsy;s father of poisoning. Booby Trap starts off with Wolfe complaining that Goodwin has a grenade - pink, cylindrical, 7 inches long - in his room. The grenade later figures in a death which Wolfe solves.