Reverted flue/vent

I recently came across an old coal-fired, gravity warm air furnace which had been converted to a gas-fired unit with a blower added. The original unit had a "reverted" flue or vent. Can someone tell me exactly what the function of a reverted flue was on the original unit? I believe the furnace was a Sunbeam and the gas conversion unit was a Waterbury.

Thanks.