Yes - you really are reading an article about dust bags, and I really have written one. I can only reflect that Filomania knows no bounds!
It’s quite common for high-quality, leather items to come with a protective bag for use when storing or transporting them. Typically the material will allow some air permeation, to help avoid mildew etc. As well as keeping the dust off, a bag helps to prevent dents and scuffs on the leather.
However, I’ve found such protection is less common for Filofaxes.
Back in the 1980s some of the most expensive models did come with a “protective felt pouch”. This one contains a 9KLF “Grosvenor” kid leather wallet.
At some point I acquired another bag, this time with a drawstring. I don’t know when it dates from or what model it would have been supplied with. If anyone can provide insight then please do so.
What options exist on the market today? For personal size (which I use pretty much exclusively), I’ve found that cotton drawstring bags 20 by 24cm are ideal. I got these ones on eBay recently.
They are large enough for a 5/4” Winchester, as shown here. I think you would need a bag of the next size up for an A5 Filofax.
There are a variety of bags at reasonable prices on Amazon too.