Update: Specifically for those who want to book a timed-entry ticket online PRIOR to actually buying your PMPass:
I reserved my first set of tickets just a few days ago. I made a second attempt today to reserve 2 tickets because I didn't trust the way I did it the first time (printout was generic-looking--although from the official Louvre website--no barcodes, neither of our names, etc.)
Indeed, I got completely different-looking tickets this time. I don't think my first "tickets" would have been valid.
When went into the FAQ section on the Louvre's website (www.louvre.fr) it states that in order for tickets to be valid, each ticket MUST state the ticketholder's first and last name clearly, and also show a barcode. (My first tickets had none of this--in hindsight, it looks like it was just a printout of my in-process order showing what was in my "shopping basket" ).
Here's how I booked them the second time-- at no charge, with both of those requirements and without having our PMPs in hand:
First, make sure when you create your personal account on the Louvre website that you include your telephone number on the page that asks for your basic info: name, address, etc. --even though there is no asterisk indicating that this number is required.
Once your account is set up and you are ready get your timed entry ticket, go to home page and click on "Buy advance tickets". Scroll down to "Paris Museum Pass holders" option. (Don't click on "Individual Tickets" option, which won't get you what you need and requires payment.) Go directly to the "PMP Holder" option--even if you don't intend to buy your pass until you get to Paris.
Then pick the date, time, and number of tickets you want. Then they'll ask you for the first and last name for each ticket holder, as well as a "N de Series". Obviously without having your pass in hand yet, you won't have a series number to enter in there. But when I put my cursor in the empty box, I noticedto the right of the box was a little head with a down arrow. When I clicked on that, the only drop-down option was actually my cellphone number, so I clicked on that, which it accepted.
I was then able to proceed to submitting my ticket order. I downloaded and printed out tickets with our full names and barcodes on each one. These tickets will then be scanned along with the presentation of our PMPs. Which we will actually have when we get to the Louvre!
Hope this is helpful to someone out there .I just happened to go into the site to see if there were any changes, and in going through the motions again, completely stumbled into being able to submit my order without a PMP serial number.