Without knowing, it is possible that if the board sequencing logic is only configured for 6 pulse ,
that configuration will not work for single phase.
Looking at the 4 images, they only show either a 6 pulse rectifier, or a 3 phase AC to AC controller.
And as there is no input for a separate phasing transformer, probably phasing info is obtained from the cathode connections, with all 6 being needed.
Maybe someone can translate the notes in the images.
There is no market for a board like that for single phase as it is no longer used at high power these days.