There's a lot of speculation (well, three people) about whether the rough side goes against the clutch disc/plate or the cam/rotor.

I updated the bingo site to say this:

I'm now leaning in the direction of rough side against whichever surface you want to slip. Seems like the rough side provides less resistance. This is kinda the opposite of what you'd expect, but the theory is that the rough side exposes more oil on it's surface, so it's got more slip.

When using leather belts to drive pulleys, you put the smooth side against the pulley surface. The smooth side grips better.

A service bulletin and update for Frolics and Bally Beauty (now on the site in machine listing) changed some cam switches and upsized the clutch washer and plate. The new switches required less force to lift, and the larger clutch surface meant less clamping pressure needed, so less wear. In that bulletin, they said to put the rough side against the clutch plate.

There's a few different styles of clutch plates:
- basically smooth with a indented ring that wouldn't effect grip (above pic) - rough side to clutch plate.
- nubs that fit into holes in the washer - smooth side to plate
- plate has a hatch pattern etched into it - smooth side to plate

I still have no proof for this, but it's consistent with the service bulletin and my mood at the moment. It also makes Muggle happy since the smooth side would be more likely to stick to the bakelite and the clutch plate side would slip, so no wear to the bakelite.