The question that is raised here is an interesting one. Taking from a more “wooden translation” like the New American (Alexandrian text which is earlier than the KJV’s Byzantine TR). The confusion is really how the editorial committees of various translations handle various texts.
The text in question is Ephesians 3:12, “in whom we have boldness and confident access through ‘faith in/of Him’” Here is where the syntactical analysis can be fun or a headache.
If the rendering of the original phrase is rendered “dia tais pisteos autou” (spelling it out phonetically), here we see the preposition dia, with the respective case (i.e., genitive/ablative) which can be rendered, in many contexts “through” or “by means of.” There is the question that comes up, Where is the direction of faith pointed to?
Taking and translating the texts with no English translation help except for consultation and looking at the grammatical syntax, the last part of Ephesians 3:12 can be translation,
“. . .we have boldness and confident access ‘through the means of his faith’”
So is the phrase be rendered by the KJV or by another translation correct like the NASB? My answer is. . . Both. This is not a matter of which one is right or wrong, but the question of how the editorial committees of each translation chose to print out the text. BTW, One my favorite texts to each translation chose to print the text.
The KJV went with “by the of him” while the NASB went with “through faith in Him.” Why is this confusing? Is it the way that translations present themselves? Could it be a question of whose faith? This is where consulting the original translation can demonstrate that there is really no issue.
Honestly I lean more toward the NASB because the NT really sticks like superglue to the literal rendering of Greek text, while smoothing out word order and putting idioms in the marginal notes. I also look at Young’s Literal Translation for my translation studies, and this phrase in Young’s looks like the KJV.
BTW, I agree with @SeanO’s reference to the NetBible as well.