Funny, but pre-milled to me means the grains have been milled prior to sale to the end user. Does that make the word "pre-milled" equivalent to "milled"? I guess so. And probably equal in meaning to "post-milled", too.
After posting, it made me think of the word "cleave", which can have entirely opposite meanings, depending on usage: To cleave can mean to divide or to bind together....English is a weird language - at least to the rest of the world.