I appreciate that it resurrects a little old thread, but I thought I would add my experience.
First of all, for a group (unlike a single contact), the field [PHONE_NUMBER] has the form [PHONE_NUMBER-EPOCH_TIMESTAMP_OF_GROUP_CREATION]. Similarly, [AUTO_INCREMENTED_NUMBER] is actually [EPOCH_TIMESTAMP_OF_UPLOAD_TIME].
It is relatively easy to find the timestamp of a specific date / time.
However, WhatsApp seems to remove old icons when loading new ones. I created a test group, added an icon to which I could go in the URL format as described above, then added a new icon (accessed through its own URL), but the original URL now returns a blank page.
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