I realize this reply is over a year after the last reply, but I just wanted to add clarity based on what I recall when speaking with others on this topic and also speaking with Mr. Milholland in person. The following applies to those of us who downloaded the PDF plans. The WEB-## is not the serial number. Otherwise, many of us would have the same serial number. A good candidate for a serial number can be found on the email you received which contains the link to download the plans. On this email, you will see the order number next to the date that it was ordered. It will look like this: [Order #123] (January 1, 1900). Using this example, it would be 123. This order number is unique, so it would be a good candidate for a serial number. I've attached a copy of what I received but altered to match the example above.
That being said, under 14 CFR Part 103, ultralight vehicles in the United States are not required to be registered or to bear any markings or serial numbers, so if you choose to use one at all, you are free to choose whatever you wish. Another method is to choose a number that represents the year, month, day, and time (down to the second) that you write it down. For example, I could use 260716075317 (July 16, 2026 at 7:53 and 17 seconds in the morning) as my serial number and it's highly unlikely (to the point of near impossibility) that someone will follow the same convention and pick the same exact point in time.
And yes, I'm well aware that I am a nerd. My wife makes sure I know this on a daily basis.
