Need a crossover for my new 400 lb center channel speaker...

[bowtie427ss]

but I don't have any 15067s,

You looked at least a half dozen or so in the eyes while you were here. Lemmee know if you go that route, i can hook you up.

Anyone else know a source for a similar late 40's early 50's N500?

Ebay, where they average around 500 a pop for singles. You'll either have the good fortune of crossing paths with one in the wild(which could take way too long), or build a facsimile.

Aren't those early networks a "series" topology?

Considering the resources available to you, why don't you have one built yet?

Fiberglass is as nasty as it's name implies.

I understand some people have a nasty intolerance for glass. I remain thankful that i'm not one of them, as it's the best speaker damping material and home insulation available to mortal men. It's also great for controlling room acoustics.