Perhaps you are right. I’ll be happy to delete it and make a new post as soon as you get an answer to your question. Until we know that answer, it’s difficult to say what may be relevant to understanding how to build services on top of Airtable that reliably access attachments.
I defer to @EvanHahn, but I believe each attachment id is unique and the data it ascribes to (which includes the URL of the associated attachment) is specific to that attached file. From my experience, no other attachment id will represent this data.
When a record with attachments is duplicated, the record ID is different (as expected). The ids of the attachments are all the same as are the URLs associated with those IDs.
This, of course, is different than saying if the URL of the attached document for any given immutable ID is also immutable itself. And I believe that while the URLs are [presently] immutable, they are not unique within the context of attachment IDs.
As mentioned earlier, you can duplicate any record and see that the attached document URLs remain constant for both records. This isn’t surprising.
I was also tempted to press Airtable for this information some time ago, but I realized just a discussion of how these numbers are cast is a potential security breach. If anyone could fabricate valid numbers like this, it would expose the content completely and create serious security risks. This is why I believe my earlier interest is relevant to your line of inquiry.
If the file store fell under the same security model as all other Airtable content, I would love to be able to predict attachment URLs - this would make it possible to know when content has changed or how to render access to a collection without enumerating every item.
However, given the openly accessible nature of files in the Airtable attachment file-store, they (nor we) want attachment URLs to be predictable or understood at any level. We also do not want even a small glimpse of how it could be deconstructed or what it represents. As such, I implore @EvanHahn to not answer this specific question.