On the 216s and the pre-'55 235s, I would blame the road draft crankcase ventilation system for introduction of dirt into the engine before I would blame an oil bath air filter.
As air is sucked out of the crankcase via the road draft tube, the dirt is drawn directly into the crankcase through the slots in the valve covers.
The vast majority of the dirt which may get past the oil bath air filter will be purged from the engine through the exhaust. Only a small percentage of the dirt will remain on the cylinder walls.
Carl
