THz, that's just cotton covered magnet wire if I'm not mistaken. I'd be surprised if it's still available.
Given what the instrument is, what it probably cost new, and that these resistors range from 500 R to 5K, and that the circuit is a precision resistive bridge to measure glass photodiode current by nulling, I very much doubt they are ordinary magnet wire.
Temp stable resistances were often wound from Constantan, a wire similar to nichrome.
Ah! Constantin. Yes, thanks, that's the word I couldn't remember. Next question is whether that wire in any suitable form is still available. Having the keyword helps.
If one will power up it might be a good idea to measure the current and voltage across it and let that guide your choice of replacement resistance wire.
Actually due to the circuit arrangement, with stuff unplugged all these resistors have at least one end isolated, so can be measured directly. Determining their values is not a problem. And I guess I can hot air them while measuring, to roughly estimate temp coeff. to see if it's some ordinary wire or something like Constantin.
"See if it will power up" - Heh. The entire thing runs on two D cells. Directly. Though the photocell circuit actually has some electronics, the rest is ultra simple. Except for being a tangle of connectors and a multi-pole, multi-position range & mode switch, that makes converting my traced circuit mess into a sensible schematic, non-trivial.
Today's unrelated question: does anyone have data on these old hockey-puck SCRs? (See pics)
I was just given three SCR & inductor modules. Two of the SCRs are marked westinghouse T920100803DW 7526.
The third one looks physically identical but is marked PSI 1000 10 MJA 2.
The round end contact faces are 44mm dia.
I'm hoping to find data sheets, mainly for curiosity. Judging by the medium degree of surface corrosion they are probably quite old, so nothing special specs-wise.
Slight bummer: The one I took out of the tensioning frame to clean, the number rubbed off the ceramic with the very first light wipe. It's apparently just black ink on glass-smooth ceramic, as opposed to glaze baked onto the ceramic. Dammit.