Here's a look at the back side of that apron. You can clearly see that thick ribber piece sewn to the apron itself. Apparently it created enough space to allow the developer to flow freely and in even contact with the emulsion. I used to think those little dimples were pinholes to circulate the dev, but they aren't. I guess they just discourage the film from sticking to the apron. My apron is very old and brittle--it's got several cracks in it, so I've never used the tanks except as decoration. One tank, though, is labelled for pack film and has a rack that puts the film sheets in a taco-style configuration. That one might be useable.