If we are using the word “guarantee” in this context, then yes because it would be impossible to sell out of an edition during the matching pre-order period. In that way, you are guaranteed a copy, and we will match.
If however, you wait for the public pre-order, provided you are an owner of record for the previously announced book, then we will still match your number automatically. You also do not need to know what your number is. Simply place the order (preferably using the same email address you used on your previous purchase), and we will match. It goes without saying that if you wait too long, and the book sells out, then we cannot match anything, because there is no book to match to.
One exception to this rule is that if an edition remains in stock for some time, and you wait too long to place an order, there is a chance that we have already assigned your number to someone else. In this case, we will indicate the cut-off date on the product page.
Lettered editions tend to be different in that it is very rare that copies will go on sale following the Matching Pre-Order period. If you do not order a Lettered edition during the Matching Pre-Order, then it is unlikely that you will be able to match later. Typically any available copies will be sold on a lottery basis.