Originally Posted by
Patrick Chase
Joel Moskowitz
claimed in his blog that RI used D2 because Joel asked him to (about halfway down, para starting with "A few discussions"). Joel also states on the product page that he originally wanted A2 but chose D2 because it was more available in Sheffield, which is probably what you saw.
Those chisels are effectively a "bespoke product" for which Joel (TFWW) has historically been the sole distributor, so his recollection seems credible.
Found it:
"Regular modern carbon steel would be okay, but we thought that the extra toughness of A2 might be better. But in England, where these chisels are made, A2 is hard to get in the sizes we needed. So we decided on D2, a slightly more expensive, more durable, overall better tool steel."
I've not read his blog yet but did just read the slight back bevel recommendation which I had forgotten.