I am asking this basic question because I picked up a a block of 4 with a vertical gutter that was being sold as a 576. I bit because it was one of those late night eBay
auctions and I got it for very less than $3. I suspected it was really a 631 gutter block, and although I have not measured it yet, all other evidence says it is 631.
But this got me thinking, why do I not see any gutter pairs/blocks for 576, like I do for 631? I've done looking and reading, but nothing pops up. Curious as to people's thoughts.