yeah, maybe RG316, I can believe that.
It's not RG400. That's what I use for making my "QRO" baluns, as unto the K9YC cookbook referenced above. You need a 2.4 in dia ferrite to handle enough turns of that, and then they are big and heavy. Plus, you can see in the photo that the diameter of the coax is comparable in size to the center pin of that SO239, so it's smaller than RG400. Incidentally, using all new materials, RG400, connectors, and cores, I can build a choke for about $25-$30.
thanks,