Arbour pin question (not about pins and blind holes)

[H Wally]

Just curious, what's the purpose of the spiral cut into the arbour pin on the 1849 colt pocket revolvers?

I would guess it's to clear crud that would otherwise bind the cylinder, but the spiral goes counter to cylinder rotation so that doesn't make sense. Plus it only goes from just under the front of the cylinder to halfway back.... any ideas?

 

[oldiron]

I think it's to keep oil or grease in for rotation.

 

[Waba]

I'd imagine that a fouled base pin with its spiral grooves consistent with cylinder rotation might tend to apply forward pressure to the cylinder during operation. The counter direction you noted would promote smoother action.
It has always been my understanding that the grooves were there to reduce the effects of fouling.