Let's look at the PINCH today. The PINCH is something borrowed from incandescent lamp manufacture and is where lead-in wires pass through a valve envelope in a cleverly wedge shaped piece of glass which not only provides a perfect impermeable seal but also provides a very nice mount upon which an ELECTRODE CAGE can sit.
As can be seen from the photo sequence below, as we work from A to C, in A you can see the sectioned PINCH at the base of the envelope showing clearly the transit of the wires through the impermeable glass seal. In B we can clearly see an electrode cage being mounted atop the PINCH. In C we can see moving from L to R, the support structure and thence the cathode being added as the mounted electrode cage is successively built up.
This next photo shows the semi-molten glass stem being made into a PINCH on a Mullard glass blow-form machine.
So, there we are, we now know all there is to be said about valve PINCHES!