Earlier quoted context omitted.
That's actually kinda hard, no? Say the server has a counter. When you load the page, it's at 57, so it displays that you would be ordering #57. While you're looking at this, someone else loads the page - what number do you show them? If you show 57, then whoever orders first gets it and the other person gets a message "Sorry, not available. Want 58 instead?" but the same thing could then happen to them with #58, too…
Exactly why we kept it simple. You find out your number when it ships.
What guarantee is there against transit hash collisions?
When two rocks are ordered, several rocks arrive, some with no ID numbers, others with IDs that match previously issued rocks?