Answer: When Schneider's published literature says the accessory thread of the Center Filter IVb is 112mm x 1.5mm.

Explanation: Two B+W 112mm filters I recently acquired measure 111.9mm across the 1.5mm-pitch male threads, which is close enough in my book given they're surely not cut for 100% engagement. They will not screw into my CF IVb, although they will screw into each other in either order (implying both male and female threads are good). The Center Filter's outside diameter is almost exactly 112mm, meaning its female thread is necessarily less and thus too small to accept standard 112mm filters.

It's surprising that a firm that's been around as long as Schneider would make such a doofus error. And I won't even mention the 112mm Kenko filter that has 112mm threads, but with 1mm pitch - what were they thinking? Thankfully I have alternatives. And I get to sell the useless-to-me 112mm filters - it always makes my wife happy to see the occasional box leave the premises.