I use this opportunity to first introduce myself. I ‘am 36 years old and live in The Netherlands (sorry for the bad English). I started working in the street lighting industry when I was 17 years old, as a maintenance technician. Today I write and oversee major maintenance programs. I have many sox lamps removed, replaced by LED.

A colleague of mine is retiring soon. I would like to give a keepsake to my colleague in the form of an ‘old’ sox lamp, equipped with small LED lighting on the inside. A kind of memento of the ignition and light of the 'old' lamps. For that I want to open/dismantle an old sox lamp and put small LEDs in the inner balloon. I may want to frost the outside of the inner balloon for best results.

Main question: How do I safely dismantle a sox lamp?

Second question: How do I remove the lamp base (and release pressure) without breaking the glass?

The safety data sheet indicates that sodium and lead are present. “Lead is found within the glass tubing inside the lamp and in de base solder”, it also states “particle diameter < 1 mm”. For the solder, I get that - that's nothing new. But what else should I pay attention to?

In the innertube, in the dimples and somewhere on the glass, I think I see lead or sodium deposited. Which of the two is correct? I would not want to remove that material further and leave it for what it is. By my understanding the sodium is present in gaseous form. It will react when it comes in contact with water, but is otherwise non-toxic as far as I know. Is my knowledge correct?
 

In short, I wouldn't dismantle it and instead run it pure with the correct control gear. Sodium is solid at room temperature and rests in the dimples of the inner tube. It relies on heat from an electric arc in neon to vaporize the sodium to produce the golden light (hence the red light when first switched on).
There is a vacuum between the inner and outer tube , breaking the base or glass will likely destroy the getter (the black at the base of the outer tube), turning cloudy white and expanding up the tube. Breaking the inner tube will allow the neon to escape and air to enter. The sodium will then react with water in the air, covering the inner tube in white crystals. It is not a good idea to dismantle a sox lamp.

If running the lamp with control gear is not an option and an LED replica is the only option, your best bet is see a professional glassblower and ask them to replicate the outer & inner tubes.

Agreed. There isn't any lead worth talking about in a SOX lamp, but the metallic sodium will react very strongly (!) with any water it may come into contact with, which is why the disposal instructions recommend breaking the lamps up in a bucket and hosing them with water from a distance..!

As a tribute to a friend and colleague, though, a working lamp on a display board rather like this would be fitting, I think 

Welcome to the site from me and the rest of the administration team, by the way 

I think the best way to open the SOX lamp is to cut open the glass just above the metal cap.  If you have a diamond saw or carborundum disc of e.g. a small Dremel cutting tool, that is relatively easy.  But wear safety glasses and be prepared for a bang when the vacuum is released and air sucks into the lamp!

Then to get out the old discharge tube, since the two limbs of the U-tube will be wider than the hole around the cap, you will have to crack it.  That can be done by squeezing the two ends together so that it shatters the glass around the U-bend.  There is also a large diameter mica disc supporting the ends of the U-tube, but mica is a fragile material and should be possible to grip it with pliers and pull it out.  The dimples in the glass contain pure sodium metal, which is moderately dangerous if it becomes wet.  Best to crush the glass into smallish pieces in a bucket and then spray water into the bucket from a distance - after a few minutes the sodium will then be neutralised and safe to throw away both the water and remaining glass.  There is no lead to worry about, that was removed from the glass several years ago.

NB depending on how far you live from Tienen in Belgium, I have a water-cooled diamond saw in my lab where we could make a much neater job of cutting the glass open - if you are close enough to want to make use of the facilities at the Sylvania R&D labs here.  I also have a stock of LED Filaments in orange colour that look very similar to SOX, and some tiny DC electronic drivers that would fit inside an E27 lamp cap.  It could be interesting to try to make an LED filament orange colour lamp in SOX shape, that would work direct on 230V!