Every 10 minutes the new review count and overall total refreshes. Consistently it either holds steady or ticks upward...
Save for one peculiar instance at 23hr today.  
   
The figure plummeted by upwards of 1,000.
Has anyone else spotted this anomaly?

I don't think most people here would sit around and watch counts change.  

I know most of us are in the US, but this is an international website. He could be anywhere in the world.

a military base.

I doubt someone was sitting around watching all day and noting the changes manually. Probably enough people here that could make some small monitoring bot (or know enough to ask one of the AIs to write the code for them) to monitor and log.  

what the new review count was 10, 15 or 20 years ago.  Anyone know how to get that?

I think it was sometime around 10 years ago reviews hit the 1 million mark -- IIRC there was even a poll/competition to see who could guess when that would happen. Forget how much VIP was given to the winner but want to say something like a year.

I hope this was a purge of fake reviews

Yep.  I remember a contest for whoever wrote the millionth review.  They won a lifetime membership, I think.