If you have an oscilloscope probe with a 10:1 setting you can use that and multiply by 10.
No danger with 300VAC , I have never seen s probe that would not withstand that Voltage - at least as long as it take to get the reading.
Kurt
There was a thread about this company and these resistors some years back. A person that used to work there replied to the thread using the name "Electrosil69" on Jan 20, 2013 - I would like to get in contact with this gentleman if possible.
Thanks Kurt Lilienthal