Strider is likely right, or there is another (isn't there always) odd designation for some other 13% Cr grade of stainless. 410 wont hit your strenght (unless the 17-4 was going to be really soft), and the others wont have the corrosion resistance I'd think you want.
I would reply to the supplier that you requested 17-4 and thats what you are going to buy.
Is this a small quantity? McMaster Carr has 17-4 bar I believe.
Other wise time to find some other suppliers.
Nick
I love materials science!