Looking at the CD25 Sadler, the beam build-down is also slight. What is obviously the compression AREA is bracketed by wooden structures creating alcoves where lockers and heads are usually put. Haven't researched this at all, But the s/v Allia (with a good inside shot) has a #738 on her sail. That is a surprising difference in numbers of boats between the two designers if that is indicative.
If there has never been a problem with the CD25D mast, it is very worthy of study. And inspiration. We know that Alberg was a tyrant when it came to messing with his designs - the 'engineering' inside under the mast must be well thought out.
It's fantastic you have two live models to draw from.
I would like to see a cross-section and a diagram of this CD25D mastbeam puzzle. Maybe not, because I would be hard put not to change what 338 has now!
A bunch of luck with the project!