Supplier of Glass plates

grat said:

580nm

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That cutoff, in fact, is classic ortho. Perfectly safe under pure red safelight, should also be safe under a sharp-cut amber that emulates low pressure sodium. Really simplifies development by inspection, but will record red lipstick or roses as black or very dark in the print.

And another hole in my knowledge is exposed-- I knew ortho was mostly blue/green sensitive, didn't realize it extended into the yellow.

Donald Qualls said:

Perfectly safe under pure red safelight

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Slavich recommends "dark-red filter No.107" (transmits 600 nm and above; maximum at 640 nm).

grat said:

I knew ortho was mostly blue/green sensitive

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"
Orthochromatic emulsions still have high sensitivity to blue, generally correct sensitivity to green and bright yellow, but low sensitivity to orange and are practically insensitive to red, as they do not register wavelengths longer than approximately 560-600 nm (medium yellow to orange).
"
http://camera-wiki.org/wiki/Orthochromatic

PFN-01T plates are probably "isoorthochromatic" (have improved sensitivity in green-yellow part of the spectrum).

My plans of visiting Pereslavl and shooting Slavich plates with Fotokor in this beautiful 870 years-old town are postponed indefinitely. Sigh.