The reference to “Grade 55” is for getting a stainless steel all thread rod with a minimum yield strength of 55,000 psi. There are currently no stainless steel specifications commonly stocked that will guarantee a 55,000 psi yield strength. The most commonly stocked grades of stainless all thread rod are A193, A320, and F593. The classes stocked in these grades require a 30,000 psi minimum.
However, due to the strength gained from work-hardening during the cold thread rolling operation, many heat lots of stainless steel all thread rod 1” diameter and under will exceed 55,000 psi yield. If you require 55,000 minimum yield in these diameters, you will need your supplier to check individual heat lots available at the time of purchase.