Answered

I believe I mentioned that I bought myself the entire Little House collection on e*bay, yes?

Well, as I was reading it, we came upon Laura's 5th birthday.  She was spanked 5 times...once for each year plus one to grow on.  So, now we know it was not just a custom in my family, and it goes back at least a hundred years before my birth. 

In fact, I think Baby Carrie Ingalls was born about a hundred years before me (106 according to Wikipedia).  So, there you have it.  Mystery solved.  (We just finished the chapter on the sugaring dance. I've never read Little House in the Big Woods before so I am enjoying it as are my husband and sons.)