509-1 Optimum Replacement Fire Bricks #5-8. Vertical Brick set for inner layer of the firebox Sold as a set. High quality bricks. Other bricks are 1-4 for the bottom. For instance, 9-12 for the upper outer layer are a different set. Bricks are cut to fit. Therefor, they are all the same to fit the spots needed in the fire pot. Other bricks can work if you want to cut your own out of a standard fire brick.
A good article on fire bricks can be found here. https://en.wikipedia.org/wiki/Fire_brick There are a lot of different configurations of fire bricks. Some are light and porous. Although, some are heavy and retain heat better. That's what our bricks on the Optimum are. Hold in that heat for better burn in the fire pot. https://flameinnovation.com/product/full-size-uncut-bricks-9-12/ Different Optimum Replacement Fire Bricks.
The standard fire brick measures 1.25 thick x 4.5" x 9" normally. This is what is referred to as a split brick. Fire bricks for kilns are generally bigger at just over double the thickness at 3"