This is the first chance I've had to look into this further. I went and checked my set of original FBD filters for my Duplex. I purchased my unit new before they were discontinued so I know they were OEM pieces. They showed no indication of Roscoe product or serial numbers. Could it be that the construction of the filters changed over the product lifetime of these safelights? Mine was purchased quite a few years ago.
I then had a look at the spectral transmission graphs made available online by Roscoe. There are two very slightly different versions of the #19 Fire filter listed. I've referenced the version I purchased. Here are the three in question for others who may not have seen them:
Rosco E-Colour+ #019 Fire
Rosco Roscolux #3406 Sun85
Roscoe Roscolux #3407 Sun CTO
And here again is that LPS line spectrum from Brian Niece, Associate Professor of Chemistry at Assumption College in Massachusetts (fourth item down the page):
Low Pressure Sodium Line Spectrum
From the generated line spectrum of an LPS lamp the extra blues and greens caused by the inclusion of the argon/neon Penning mixture are readily apparent. And a look at the #19 Fire filter chart shows that it should be almost perfectly opaque to those wavelengths all by itself. For standard OC-rated b&w materials this filter should work fine. And in practice, at least for me, it does.
On the other hand, I'm not sure what additional safeness the other two filters would add, given their respective charts. Especially in the green regions for VC papers. Filter #3407 in particular allows a substantial percentage of blue-greens and greens to pass. This should be an obvious risk factor for VC papers if the #19 were not present.
Might these second two filters be intended for different materials? I'm really curious regarding your observed results.
I purchased a set from Freestyle while Thomas was still in business. I bought a yellow tape, a red tape, and a black tape. I took them apart a few years later to see if they were filters I could replace myself for less money. In the yellow tape filter I found two sheets of gel filter material. One had the number 3407, the other 3406. I recognized them as Rosco filters from my sample book. The red tape had "ROSCO 19" printed on the edge of the filter. The black tape did not have any markings, but were the same colors as the yellow tape, but used more layers of filter material inside. All of them used the tissue paper that comes with Rosco filters as the diffusion material.
The numbers were not present in both filters of a pair. I don't think they were intended to be left on the filter material when cut and put in place. I had a very old set of Thomas Duplex filters, and they were different material. It was far too old and faded to tell what they once were. It is possible that the filters sold for these safelights now and just before Thomas Duplex stopped selling their lights are made by someone else and they just use what they think may work, but if you buy a set of filters from Freestyle or KHB Photographix, these are what you get.
You could always buy a set, open them, and tell us what is in them. If they are different than what I have, then I was sold the wrong filters. But I doubt it.
Before my successful (pre-fogged) safelight tests I fired up the Duplex, let it warm for ~30 minutes, then looked at the CD reflections unfiltered. In addition to the massive yellow sodium doublet, I saw the much, much fainter blue and green spikes. I also saw the extra deep red spike.
Then while looking directly at those spikes I interposed a single layer of #19 filter. The blue and green spikes disappeared. When I removed the #19, they returned. Back and forth.
Later, when I performed the safelight test I was able to leave a sheet of Kentmere Bromide #3 graded paper out for ~30 minutes with no hint of fogging.* As you may know, bromide papers are very fast. It should have responded to any blue light present. But it didn't. It remained pure white. (Even checked with a reflection densitometer to factor out my subjective eyes.)
So as I said, I too am confused.
Also keep in mind that if you use a cover sheet of Rubylith you will likely be filtering out all of the yellow sodium light as well. At least in a perfect world. I've never tried it myself, so I don't know how much will pass. Maybe only that secondary red spike I mentioned above?
* Apologies, as I earlier identified the test paper as Ilford MGIV. It was actually the Kentmere Bromide. The Ilford was used for my red LED safelight testing.
In regards to the paper that you left out for 30+ minutes, did you flash it at all to see if there was any effect on top of exposure? My tests didn't show much, if anything, in blank white areas. The degradation was in areas that were already exposed by the enlarger.
Yes, I did. That's what I meant when I said "pre-fogged." Perhaps not the best choice of words by me. Probably should have said "threshold fogged."
A test was first made to determine threshold exposure for the Bromide #3 paper. Then the test sheet was pre-exposed to just below that threshold level. Any additional safelight fogging exposure should have therefore been immediately visible after development. Either to my eye, or to the far more sensitive reflection densitometer. Comparisons were made against a similarly pre-exposed control sheet that was not subjected to the safelight.
It should also be noted that 30 minutes was an arbitrary cutoff by me. Because there was no detectable fogging, the actual safe time limit was therefore an unknown time beyond 30 minutes.
When I later performed similar testing for a DIY red LED safelight under Rubylith, I extended the test out to 60 minutes without any apparent fogging on MGIV. That's where my earlier incorrect reference to MGIV came from.
I simplified the solution in my 10x10' darkroom. I use the supplied yellow/tan filter over the tube. I inserted black foam core in the vanes to attenuate the light. I have never had a problem with fogging in the 20+ years I have used this light.