SIS downtube shifters - ghost of 600EX in current Dura-Ace?

I understand that the old 6-speed 600EX shifters were
notorious for shearing off the alignment fingers on the index unit
that engage the Lever Boss Cover. I've experienced this failure
myself, and have looked at later Ultegra levers, as well as a 6-speed
105 lever and see what Shimano did about this flaw. The index module
is assembled with the lever captured in it. The base of the index
module is now much larger - not having to slip through the the lever
from the outside - and engages the large lever stop of the boss cover to
hold the module against turning, instead of relying on the little fingers.

However, I find myself a little puzzled with the Dura-Ace
shifters that seem to have been made since that time. First, Loose
Screws lists an 8-speed Dura-Ace index module:

http://tinyurl.com/cbq29

suggesting the captive lever design used for 105 and Ultegra,
wasn't used for Dura Ace. Shimano's exploded diagram for the current
10-speed lever also corroborates this:

http://tinyurl.com/d2ycx

This index module looks like it would fit perfectly into a
600EX lever! Presumably the 8-speed module would fit too.

But, this leaves me wondering - presumably Shimano learned
their lesson, yet the Dura-Ace index module appears to depend on the
same tiny fingers that were 600EX's achilles heel. Presumably
_something_ is different about the new units to prevent the failures
seen of the old ones, but what?

I can think of two possibilities. My first guess would be that
the Dura-Ace module was built on a steel core where 600EX used some
weaker alloy. The other possibility to cross my mind is that the core
engages the flats on the tip of the shifter boss, and supports its load
there (600EX's front shifter does use these flats, though 105's
light-action front does not).

If anybody is more familiar with these levers, I'd be
interested in hearing any insights, even if it's as little as vouching
that the current product isn't as failure-prone as the 600EX units were.

-Luns