We helped introduce and develop this board when Roy ran the company.
It was done for the Sieg X3 machines that had mains voltage on the spindle side of the board.
The original board was the V3, later to become the V4 that had an optional 2 relays fitted.

All these boards are isolated by means of a DC to DC converter.
The manual for the V4 is still on the web.

www.diycnc.co.uk/forms/spindleV4.pdf

One thing to note:

On the V4 Roy made a bad choice when he chose the DC to DC converter in that he chose one with a low amperage. This showed itself when we built a control box that was 6' away from the machine for a disabled user and speed just didn't work.
We found out that if the DC to DC unit was replaced with a higher amperage the problem went away. Ironically the 2 amp unit was cheaper than the 1 amp unit.

This was reported to Roy but ignored.
Together with some other problems it's one reason we stopped buying his boards.