Toyo 45D how to remove rear standard

[ac12]

I have a Toyo 45D with a cracked mount at the bottom of the rear standard.
To repair/replace it, I need to get the mount off the rail. But to do that I first need to get the rear standad off the mount.
When I turn the focus knob, the rail goes front/back till it hits a stop, and that stop keeps me from getting the standard off the mount.
Can anyone tell me how to get the standard off the mount?

thanks