Hello Iura..
Are there any other numbers you have that are ferrite for the same size? I wound the first toroid with a T50-6 core and it is working so far... Didn't see any measurable drop in the output voltage.. getting 360-380 volts output at the capacitor for discharge.. Hmmm..
I know ferrite is the preferred material..just thought I'd try these. Size is .500 OD .300 ID and .190 thick...same size as I used before.

John