Servicing a 1950's Sheaffer Snorkel fountain pen

Recently – concurrent with my last uni resit exam that will determine whether I have a chance of ever getting a uni degree - I managed to find my ideal Sheaffer Snorkel on eBay after many, many months of fruitless searching. It had to be black, it had to have a steel Sentinel cap, and it had to have a medium nib, because my handwriting can’t carry off a fine. It also had to fill! The following posts recount this noob's first encounter with replacing the ink sac and seals in a Snorkel.