Nice Job Tony! I have masthead envy!
Since you are going to the trouble of fabbing a new masthead, you can give yourself the luxury of good attaachment points for your kites and smart access (for a tricolor, & instruments perhaps?). I know I am typically against electronic toys, but proper room for them makes the idea very tempting!

Re: your question; The additional numbers are post manufacturing processes for the alloy... You should have no issues at all with corrosion.
For reference:
http://www.alcoa.com/adip/catalog/pd...Alloy_6061.pdf