In the bed of the Elky's (mine is '68), there is a cover plate in the floor of the bed, all the way forward. There are approx 24-26 bolts holding it down. If you unbolt it and remove it, you will find quite a bit of room in there. You will see the little "pass-thru box" which goes from the interior of the car into this area. This "pass-thru box" is about 20% of the rest of the compartment. So if you pull this box, you now have access to the "smuggler's compartment". It is @ 1/4 the size of the entire bed.
I believe this is the area normally used by the back seat, when this frame is used for the station wagon. This is my guess, but don't know for sure.