That worked, thanks Tom and David! (or David and Tom),
Also, regarding the placement of the GP Layout control box
For the CPT that was created from me placing a code snippet into my site-specific plug-in, the control is placed in the main body area of the post as shown below:
https://drive.google.com/open?id=123M3wTuPzQsNTwXfi9NMEGUV8U_Rs0v5
Whereas for the CPT that was created from using the CPT-UI plug-in Tom recommended to me for testing, the control is placed in the side section area of the post as shown below:
https://drive.google.com/open?id=1G7CUGeEeOpFfRvCdTGBYxNkhUAKhWWmN
Is this (a) pointing to any potential problem, and (b) could you say how I can move it to the side section which I prefer.
thanks again,