The magazines won’t even begin to fit in there with the extra piece inserted. One might fit without it installed. I think the same is true of the single shot tray. But, you need a screwdriver to open the compartment.
Mystery solved! I sent an email to Daystate technical support who was able to tell me exactly what this was for. Here is their reply:
"The grip is from FAB defence and the insert is for their proprietary firearms oil bottle when used with high power semi/fully auto rifles and so not applicable for PCP, but they left it in anyway.: