Laurie,
This is mostly my speculation but based on excavated examples I've seen mostly in Virginia, but at 17th through 19th century sites. Most "Double D" type period buckles were rather utilitarian (even though they may or probably were a little more expensive being made of brass rather than iron) and were used on some civilian and most military accoutrements. Iron seems to have been the preferred tongue material for military buckles.
Not having a "set" area for the tongue allowed the user to use a tongue of any size he chose or easily made. Then he punched holes to fit the tongue or tongues. That's the key, I believe. Instead of worrying about making a tongue fit a pre-determined ring groove on the buckle, you made the tongue so it would go around the center beam. Also, for cartridge box buckles where they made two tongues for each buckle, casting or filing a ring groove for each would not make sense.
Gus