That large black barium getter deposit in new LPS burners certainly marked a change in the way Philips made those lamps. I also like the cleanliness of early 18 W SOX lamps produced from 1977 to the early 1990s, but heating the electrodes past their activation point -what causes that blackening- increased the burner purity significantly, which extended the lamp's service life. So, there was a use for that barium getter indeed, but the process used for its formation became unreliable in the early 2000s due to funding issues, resulting in a too wide spread in burner getter characteristics. We could say that this was the point when things started going downhill for SOX lamps.

By the way, your lamp was made by Philips in Hamilton, Scotland. Production of SOX lamps was transferred to Philips when Osram's Shaw (England) lamp plant closed down in 2000. The agreement between the two companies was that Philips would produce lamps with Osram's specifications, hence the lack of sodium-retaining dimples and the use of a graded IRC mirror on the outer jacket.

Interesting you mention the graded IR coating Max. Having looked and compared Philips and GEC SOX lamps I find it difficult to observe any visible differences in the outer IR coatings over the lamp surfaces. This may be as expected not being able to see in the IR end of the spectrum of course, just thought it may be possible to see some variation.

I don't really mind the gettering in the arc tubes myself, but it's interesting information as usual, Max