Sounds like someone wanted a D-28 in all aspects other than back & side wood, which they wanted to be mahogany. I have heard of and seen such customs before.
So you have a used Martin that says "Custom" on the neck block? And it is like a D-28 in all aspects other than back/side wood, which is mahogany?
Photos?
If it is a "custom" it is not really any given model. So why do you say it is a D-28?
P.S. Here is a link to a Custom Shop Martin based on an HD-28V, but having sinker mahogany back & sides:
http://www.wildwoodmusic.com/martin-...0&wppa-occur=2