Low-pressure sodium vapor discharge tubes have more than just sodium in them. They also have small amounts of neon and argon intentionally introduced during manufacture. These help with the initial striking of the lamp. It's called a Penning mixture. This mixture introduces additional emission spectra from the blue and green portions of the visible spectrum. (See here for a real-world illustration, 4th entry down, note the blues and greens.) These additional spikes are what is actually fogging the paper.
If you have access to a Thomas Duplex try starting it up and letting it stabilize. Then remove all of the filters so you are looking at the bare bulb. Find an old CD/DVD disc. Use that disc to reflect the light from the bulb and look closely. The disc will act as a poor man's prism. In addition to the overwhelming orange sodium light you will also see much fainter, but still significant, pure bands of both blue and green light. Once you know what you are looking for, repeat the observation with the originally supplied filters for b&w back in place.
The solution is to add an additional (inexpensive) layer of Roscoe Roscolux #19 Fire filter. It's a theatrical lighting item. It will remove all of the extraneous blues and greens while passing about 35% of the correct sodium D-line emission oranges. (See the filter technical data sheet here.) You lose about two-thirds of the "good" light in order to remove all of the "bad" light. But the Thomas is so darned bright to begin with that the loss is usually a good thing in a smaller darkroom.
The Thomas Duplex is no longer manufactured. However, if you still wish to buy a new one of this type of safelight you can (as of the time of this post) still do so from the Sebastian Darkroom Products division of California Stainless Manufacturing. Click the following link to see a description of their OC-1012 sodium vapor safelight. It looks almost identical to the Thomas. California Stainless is the OEM manufacturer of many of the items offered by Arkay.