I don't recall seeing that mount in the TM, my A3 doesn't have one, so I'm guessing your trailer may have had a generator mounted in it at one point. The 90s/2000s genset trailers had a CO2 extinguisher mount, but that looks to me to be a 10 lb ABC drychem mount.
Personal opinion: there's nothing more useless in the world than an inoperable fire extinguisher unless it's in a museum exhibit. If you're going to put one in it, please buy a new ABC so it may be of real-world use should the need arise.
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