I was rummaging around for a core for a small switching power supply
and came across some 1.00" OD, 0.55" ID, 0.39" high (25x14x10 mm)
toroids. A 50 turn coil measures about 1 mH (by measuring resonance at
200-500 kHz). This appears to indicate a permeability around 600, by
comparing it with various catalogues. But the Q is rather low
(equivalent series resistance is roughly 250 ohms at 500 kHz and 120
ohms at 200 kHz). Doesn't look likely to be good enough for a
switching supply, so they may have to be relegated to EMI jobs (which I
seem to recall they were used for in a piece of equipment I took
apart).

But does anyone have any idea what material they might be ? Doesn't at
first glance appear to be anything similar to what Fair-Rite makes.
There's a good chance it is of Asian origin, I think.

73,
Steve VE3SMA