You might want to google "billit" and see what they have to say about. My (work) experience is, any block of "raw" aluminum used to extrude "parts" is called a billit. It's possible that there are various levels of billit quality pure, blended, scrap, etc. as for the finish, I do believe that is due to 1 of 2 things....1) the surface finish inside the extruding die...(usually smooth so as not to cause tears, or binding during the extruding process) or 2) extruded products can be "finished" after the extruding process by polishing (CDA
), wet honing, etc...........
Check it out!