Without a connection between the PC/Mach3/control board/VFD, it's difficult to see how the speed could ever have been controlled from software. There just ain't any obvious path by which it could be done. Not saying you are wrong, just can't see how it could have happened!

I see that you do have a knob on the front of the VFD. There is a parameter in the HY VFD to specify that you want front-panel speed control rather than via the control signal input. That might at least get you running after a fashion.

I can see that the board output connections are labelled but most are hidden under wiring. Is it possible to note what they are? I can see a couple with "S+" and "S-". Could be the terminals that we are looking for... My own next step would probably be to put a meter across those two and see what the output voltage is (if any) while changing the Mach3 spindle speed via the MDI. I would hope to see 0-10V, proportional to speed setting. But knowing how the rest of the terminals are marked would be useful - as would knowing what the board itself was!