Yes, I've worked on that type of coupler. It's essentially the same as the newer style.
By the looks of the surface rust, I'm sure it's stuck pretty good.
Generally I like to remove the coupler from the ballscrew side first.
Are you removing the ballscrew? Or just the motor?
If you don't have the Z-axis blocked up already,do that first then loosen all the bolts on the ballscrew side of the coupler, then using the handwheel bring the Z-axis .010-.020 above your blocks. If it doesn't separate on it's own at that point, a light tap on the coupler should do it. WD-40 or some penetrating oil will help.
This method has worked for me many times.
-Luke