It worked!
I ordered a set of security allen wrenches.
Sure enough, the 1/4 inch did the trick.
I was able to remove the Dietz 650 from the barricade, and open it up to remove the corroded 6V batteries that have been in it since before the previous owner acquired it.
They were very old Ray-O-Vacs.
Too bad they were corroded. They would have made nice display pieces themselves!

I'll throw another puzzler out there, if anyone's interested.
I was taking apart the aforementioned Dietz 650 for cleaning, and was surprised that it has no screws holding the two halves of the lens together.
Does that mean this lens doesn't come apart, or is there just a trick to it that I'm not aware of?
This must be the oldest Visi-Flash that I've encountered, because I think every other one has had screws.

Thanks again,
Barry