It has less to due with the species of Ash than its weight / density. 10 years ago the instrument grade Swamp Ash being harvested and sold was (on average) much lighter weight than what is available today, so Joey and Vinny opted for medium weight Northern Ash to get the desired weight and density that they felt provided the optimum tone for their vision of what a Fodera should sound like. Remember, unless requested, tone is our primary deciding factor, not weight (within reason).
Today, the Swamp Ash being harvested has increased in weight / density to the point that it is now in our "ideal" weight / density range and that is why we use it. I am not certain of exactly at what point this inverted, but, it has been at least 5 years (perhaps more). By way of example, we just recently finished making this Imperial for Richard Bona:
http://www.fodera.com/Main/NewBornDetails.aspx?FeaturedItemID=102
Richard wanted an EXACT replica of the Imperial that he has been playing for well over 10 years. His original Fodera Imperial (which was purchased used, by the way) had Northern Ash body wings and a Swamp Ash Tone Block. To get the weight right this time around, we had to reverse that and use Swamp Ash Body Blocks and a Northern Ash Tone Block...
I hope that this all makes sense.
J