What made me assume that the receptacle is the moving part is pretty straightforward - the moving part is the only part of the zipper that you put something els

What made me assume that the receptacle is the moving part is pretty straightforward - the moving part is the only part of the zipper that you put something else into. I still don't see how "receptacle" could apply to anything else. You zip up a zipper by picking up the bottom of the other side and inserting that into the moving part. That makes the moving part a "receptacle". The zippers that I'm familiar with don't have bottom stops or retainer boxes. I'm looking at a YKK zipper on my jacket right now; there are two pull tabs, one above the other. This allows you to have the top zipped while the bottom is unzipped. (I should note that my all-American L. L. Bean jacket has the tabs on the right, and I'm inclined to say that this is true generally of American zippers. But I only have the one zipper on hand. This is another reason why indicating that having the receptacle on the left is backwards from normal might tend to indicate that the receptacle is the moving part.) Because it's obvious that the "receptacle" must necessarily refer to the moving part of the zipper (as described above), I assumed that "tab" referred to the bottom of the other side of the zipper, basically just the part of the zipper where the teeth end and the whole thing comes to a stop. I guess in your image this is the "insertion pin". I still feel justified in assuming that the "receptacle" is the part that receives the "insertion pin".