Opinions needed on this Firebrick

[summerj]

Here is the spec shet from the company. Cemex has it locally in stock.
They are 2 1/4" thick x4 x9. $ 1.75 each

Specs say low duty I thoght medium duty was prefered but that can be subjective.

What do you all think?

http://www.wgpaver.com/firebrick_pdf...20-%20BUFF.pdf