Here are the vertex distributions from the BBC scaler boards (boards 3 and 4).

Apparently, board 3 has one stuck bit, otherwise the readings of boards 3 and 4 agree within 0.3%.

The vertex distribution early in the fill and late in the fill are identical.

The vertex distributions obtained with different spin combinations have the same shape.

The left and right cut-off values are consistent with the BBC small tiles timing window, according to RunLog: values BBCsmall-TACdiff-min = 3430 and BBCsmall-TACdiff-max = 4762 correspond to 6 and 9, respectively, because only four highest bits are sent to the scaler boards.