Harry Mathias Williamson's World War I Draft Card...and an unexpected breakthrough!

Harry Mathias Williamson is my Grandmother's natural father. This is his World War I draft card.
What immediately caught my eye was his birthdate (4 May 1880) - I didn't have that before now. Also, this gives his address and his occupation (he was a truck drive/teamster, or in this case "Delivery Helper"). But the real breakthrough is also the most curious: he listed a sister, Mabel Thompson, as his closest relative. Why is this curious? Because he was married to my great-grandmother at the time. I would've expected to see her there. So why is this a breakthrough? Because by looking for both Harry and Mabel, I was able to locate their family in 1900. That simple discover has led to two additional generations of Williamsons.

But that's not all. Harry's birthdate, 4 May 1880, which was unknown until now, is exactly 1 month prior to the 1880 census (1 Jun 1880). That made it very easy to find his family in the 1880 census. Again, that coupled with a little luck gave me two more generations on that side of the family.

I'll detail what I found in another post after I organize and file all the census pages that I pulled.