What is 6511 stainless steel?

65-11 is a standard for testing mass weight of zinc or alloy coating on iron or steel.

That means the steel object is galvaniced aka covered with zinc. That also means the steel underneath is not stainless.

'Real' stainless steel is an alloy of iron and chromium (about 11% chromium).

However your mark 6511 may also refer to the item stock code.