If I remember correctly, residual moisture is indeed the main cause of the burner's internal glass staining in the electrode and seal area. However, I don't think the problem related to an improper evacuation of the burner during production, but had more to do with how the extremities were sealed. The duration of that process and the placement of the flames led to an accumulation of moisture which reacted with the internal borate lining of the discharge tube, thus impairing its capacity to withstand hot sodium during operation.

Interesting Max, in GEC lamps the brown staining would often appear almost anywhere on the discharge tube not just around the electrodes. I seem to remember James remarking that Philips didn't have this as they filled the tube with dry nitrogen prior to pumping down. I could be wrong though.

What I had in mind specifically is the local burner degradation we see around the electrodes in some lamps. But you're right, if the staining is all over the tube, then it must be a global degradation of the borate lining caused either by bad storage and handling of the tube prior to production, or/and significant humidity ingress during sealing.

Lovely old lamp, any idea of its age..? Is there a date code..?

The stamp is very faint on this lamp. Probably over handling has wiped it off. However I have managed to just make it out and it looks like a date code KF making it June 1978. This feels about right for this lamp too.

That's a nice classic. I really like how neatly molded the burner U-bend is, with those well defined sodium reservoirs, that's quite characteristic of those old GEC SOXs. Such attention to details was somewhat lacking in the later years.