Of course his next line in that is:
Ookla is right, the others don't have 1/1 correlations. But I liked this concept far too much not to use it.
In other words, it doesn't fit the pattern. When I originally read this, I took it to mean that he liked what happyman said so much that he couldn't not use it. To me it meant that it wasn't what he originally intended it to do which is why I fight on to find the original intention as this idea doesn't fit.
If its true that this is how BS intended it to work, I will be very disappointed because I was very excited about patterns and the idea of unknowns that we had to guess and if he won't even follow his own patterns then there is no reason for the scientists of us out there to even try.
To me, this isn't the obvious result to a serious allomancy student as BS indicates. There is no correlation to Internal vs External within the groupings. For instance, Brass, Zinc, Copper and Bronze are all technically external to the allomancer. Tin, Pewter are Internal, and Iron and Steel are External. Whether or not BS confirmed it, it doesn't fit the patterns.
If BS is going to say something like:
what those two metals do should become obvious to the serious student of Allomancy...
Then it is my opinion that his system should follow the pattern and maintain the 1/1 correlation.
I will keep searching for the real intended use that does follow the pattern and fight against this.