There is just as much fluidity in gender roles among chickens as there are among humans and other animals of Planet Earth. Your observations are quite good. Those hens will miss Charlie.
Four hours to the lab is prohibitive. If you saved a sample of Charlie's final poop, it would be well worth the time to find a vet who can send it to their lab for you for the fecal float and bacteria gram stain tests. It should hold up if kept cold.
Next, and this isn't going to be easy to contemplate, you can do a home-style necropsy on Charlie's body yourself. It's understandable that most people we suggest this to immediately reject this idea. I'd say just 2% are game to do it. It does require shifting yourself into a very clinical and emotionally detached state.
If you decide to take this on, it would involve a bare minimum of opening the abdominal cavity, exposing the organs, and taking a quick look for anything that appears out of the ordinary. A lot of the time, if there is something compelling to find, it's staring you right in the face, it's so obvious. Things like a liver ten times larger than you'd expect a chicken liver to be or an abdominal cavity awash in a murky brown liquid with bean size tumors floating in it, exciting stuff like that. You can get a general idea that a pathology was at work.
The fecal tests can add the scientific underpinings to what you see inside of your hen. It can tell you the name of the bacteria that killed her. Or that there was no bacteria found. It will also tell you if she had coccidiosis or heavy worm load. What it can't tell you is if a poison killed her or an avian virus, but if you find tumors when you look inside her, you will have a pretty good idea a virus had caused the tumors, but not which one.
Lastly, if you take photos of the contents of her body, some of us here may be able to spot something and recognize it to supply a little more information. For example, one very young hen that died suddenly in my flock had an impressive collection of what appeared to be hard boiled eggs in her abdominal cavity. We all learn from each other here, and I posted a thread on my findings from that home necropsy.
https://www.backyardchickens.com/threads/internal-laying-what-it-looks-like.1349959/ If you find something like that in your hen, you'll know it's something that's not likely to kill more of your chickens.