In 2e, this was generally represented with individual treasures in parentheses. But the only systematic thing I remember is that the higher letters were for individual treasure and the lower for the collected treasure of a monster lair.
I found a reference somewhere that type D was meant for dwarves (which is why they have a fairly high chance of gold), for example, but that's not explained anywhere that I know of. There's little easily discernible logic to these tables, but I'm sure they made sense to Gygax because they definitely lean in different directions.
You also had a 30% chance of 1d10 gems (to be rolled separately), 25% for 1d6 pieces of jewelry (ditto), and 15% chance of 1 potion + 2 other magic items.
So, treasure type D meant there was a 10% chance of 1d8x1000 cp, 15% chance of 1d12x1000 sp, 15% chance of 1d8x1000 ep, and a 50% chance of 1d6x1000 gp.
