Prototype set is here

[miniman82]

Yes, it's just a simple neck weld. The problem comes from the fact that if the planar dot plate and mask are heated at a different rate than the rest of the tube, the plate can crack or the mask can bend. This is the same issue as with the 15G, but it's worse because the prototype tubes have more thermal mass than a 15G does because its internal supports are heavier. This means even longer bake out times, and more cost. I've read notes from early books on the developmental tubes that state the face should be shielded during cooldown, to avoid cracking.