The seat backs in my E25 are installed with screws through the fabric as you, Dave, describe with regard to your E27. From what Missalot says, this appears to have been the norm for the E29 as well. It sounds, then, that some in the past have installed velcro so that they can access this space and thus make use of otherwise wasted storage space.
Does this seem to be the case? If so, sounds like a good idea.
Best,
Roscoe
E25, cb, Hull #226