Hi rick, i have spent a lot of time to find this material for making block, which comliance to EN12668-2 last year. finally, I find a equivalency material, and the test result of block is good. It's less attenuiation when the frequency high as 5MHz. But i have forget thei type of this material, may be you could ask the steel vendor for a equivalency material.
S 355JO means : Steel ; 355 N/mm2 yield strength, charpy impact test 0° celsius
This steel is really common in Germany ( or let me say european region ), but please have
sometimes you find S355J0G2 ( old version of 10025-2 ). " G2 " says something about in which way the rare steel is normalized !