The note taking book by Ahrens addresses this. Luhmann numbered the cards with 1, 2, 3, 4. These are the IDs. Each new card is sequentially added with the next number. Now if a card is closely related to card 1, then it can be given the ID 1a and added right after 1. The card following that can be 1b, 1c, 1d, etc. If you now need to add a card after 1c, that can have the ID 1c1. In this way you have a way of generating IDs for cards that are appended to the end, as well as for cards that are inserted in the middle of the pack somewhere. There references to a card is simply listing its ID. Once you have an ID you can look up the card in the slip box.
The note taking book by Ahrens addresses this. Luhmann numbered the cards with 1, 2, 3, 4. These are the IDs. Each new card is sequentially added with the next number. Now if a card is closely related to card 1, then it can be given the ID 1a and added right after 1. The card following that can be 1b, 1c, 1d, etc. If you now need to add a card after 1c, that can have the ID 1c1. In this way you have a way of generating IDs for cards that are appended to the end, as well as for cards that are inserted in the middle of the pack somewhere. There references to a card is simply listing its ID. Once you have an ID you can look up the card in the slip box.