Details....

The "double sided lettered pulley -- It's got text on both sides.
- Shimano on one side. W 8.6 on the side side.
- Shimano on the other side. >POW< on that same side. Also the recycling triangle symbol.
- This one has more triangular-shaped negative space from the rim to the hub.

The "single lettered pulley with a grove on the other side" -- Text is only on one side. The other side has a track/groove on it.
- Shimano on one side with arrows that must indicate the direction it spins (on the derailleur so that might get confusing).
- The other side has two circles that I thought were from manufacturing. One says... PE, I think. The other says... 10? I'm going to have to get a magnifying glass for those.
- This other side has a 7 on it.
- This other side has the triangle recycling symbol and >POW<
- There are only three holes between the rim and hub on this other pulley.


The pic above shows the A (outside, based on allen key hole? Or is that inner since it's toward the wheel/inner side of the bike?) and B, definitely outer side of the bike for the guide pulley.
C, inner/wheel side of the tension pulley.
D, outside of the bike, outside of the allen key hole on the tension pulley.


These look about the same thickness. I don't have my calipers here, but I can measure them tomorrow and get an exact measurement on the thickness between these two pulleys.


And then the metal rings that go in the center hole on these. One is longer than the other. Which is the top/guide piece? Which is the bottom/tension piece? I would guess the guide/top has the longer one since I think that is supposed to have a little more sloppiness to it so the chain goes on the cassette better. The tension/bottom pulley is just adding tension and would be fed a more exact chain position.