quarantine 'em

Mostly accurate. Hughes invented the basic concept of a "Vegas room" to act as a containment unit for harmful memetic entities, exactly as the "exterior" SCP-3125 entry implies. He probably built several of different sizes, on different sites. Some were prototypes

Some are indeed used for this purpose. (Not all are on Antimemetics Division sites.) The memory-erasing system varies in design - some cannot be bypassed. The external SCP entry varies; some give more real details than others. Some units are vacant, anticipating future need

Most likely story: the unit which became SCP-3125's was initially vacant and had no associated procedures. Someone went inside for testing purposes. There, they spontaneously uncovered SCP-3125. This person developed and implemented the containment procedures we see today

(You *can* take written messages to yourself out of a Vegas room. At first. But then the first written message says "Set up containment procedures so that nobody else ever brings written messages out of this room", and that's that.)

Interesting!!! And so the person was able to amnesticize themselves so as to not get eaten on the way out (the first time)? Also: one more thing I was uncertain about. Other Antimemetics sites are mentioned as “non-entities”. Is this because of SCP-3125, or because...

... they all ended up detonating their Antimeme bomb to stop a 3125 outbreak?

Or equally, /how many times has Your Last First Day happened?/

Going to try to keep this in one thread: yes, the person was able to amnesticise themself on the way out. Maybe the gas system had already been installed at that point, maybe there was a first aid kit with amnestics nearby