Electron lucent areas just beneath the RER limiting membrane seem to be a feature of very large and complex intracisternal bodies (ICB) (or RER SP granules if you prefer to give them a name that is what I think they may be (that is surfactant protein granules, backlogged in the RER for some reason))  This particular ICB has all the earmarks of the same layered striations seen in guinea pig and dog (this is from ferret, and is way more abundant that typically found and more curvey as well, 6911, 23303 19,500 original mag, 4 times enlargement plus whatever playing i did in Photoshop to crop and enhance contrast).

Noted here is the pale area beneath the cisternal membrane (RER membrane) which waxes and wanes around the central layered protein area which is considerably more dense than the pale areas.  And then in occasional areas where ribosomes are present on the outer edge of the membrane, there can be some streaky connections (see inset) and a little bit of increased electron density in that electron lucency.  In the inset the ribosomes, 3 of them, and streaks, 3 of them, have been “burned” using photoshop… the larger and original image was not “burned” in that area and the densities can still be easily seen (red lasso around the area that is enlarged for the inset which lies at the end of the arrow tips).