In the T&B Cat, HIGH TEMP wire marrs, they are rated for only 300V.
In the IDEAL line, they are ALL 600V rating.
In the T&B are the same in the trade size equivalent also.
The HIGH TEMP Marr's, the PORCELIN WIRE CONNECTORS are the only ones I can find, asking my supplier about this, these are the ones rated for and above 105C, these are the only marrs out there that are rated for 300V, and is the wrong type to be using in the first place in the condition in which you described as was mentioned by MDE.
I have a copy of the reference catalogue and will pass it on to you at you're e-mail in PDF for you to see for your self.
Please have a good and safe day!
Best: Robert Wagner!