This pic is under a microscope! It was soaked and unfurled. You can see faint book lungs. I don’t see spermatheca, but it is a very young sling. Thank you for the photos, those will definitely come in handy for future molts!! You rock!Soak the exo in soapy water, one drop of dish soap per like quart or so.
Spread it out, carefully unrolling the abdomen and look at the slit (epigastric furrow) between the anterior book lungs like @Teresaisrad said
You may need a microscope... You are looking for the spermatheca, which should look like this on Avicularia juruensis