Variability in the escape wheel of a 19th century Scottish drumhead

The graph below shows the performance of a Scottish drumhead tall clock. This escape wheel is eccentric and has one tooth that appears to be wildly out of place. The variability of rate with each rotation of this escape wheel is 3.4 hours per day! You can clearly see the repeating pattern of tooth spacing in this graph, which shows about eight complete rotations.

drumhead