I think that you are right and add that since the 409 starters had a cast iron nose, they did not need the extra support.My guess as to why the small blocks had the extra bracket and the 409s didn't is that the small blocks had a smaller flywheel and straight across bolt pattern on the starter. The 409 had a staggered starter bolt pattern that provided additional support to the starter. I could be way off on this guess but it sounded good to me. Of course that doesn't take into account the bell housing mounted starters.
1963 & 64 6 cyl also use the brace. I have a 6 cylinder one.348 and 409 did not have the brace. Only small blocks that I know of. Not sure about big blocks.