This is what it says on the website for my double wall. It also said to use three screws.All double wall installations that I've seen have used an adapter. The taper is different on the adapter vs. the end of a normal double wall section. The appliance adapter is smaller on the tapered end than a section of double wall.
I don't use screws to hold double wall to the stove, it is often debated here whether they are necessary on an all vertical installation.
STOVE ADAPTOR (PDAB)
Some stove collars are too thick for the telescopic length to fit over. In this case, install a stove adaptor (PDAB) on the stove. The telescopic length will fit on the adaptor.