Thinking about Mary Selkirk Frederick

Yesterday while walking about Elmwood Cemetery, this stone caught my eye. First the lovely clasped hands.  Then the inscription - Mary Selkirck. Can that be right? What is that extra "c" doing in there. I had to do some research.
 

A clearer picture I took back in 2018.

The Coeymans Herald, December 21, 1887

The death notice from the Coeymans Herald reads: 

Frederick - At Selkirk, Dec. 15. Mary Selkirk, wife of Barney Frederick, ages 43 years.  

The stone says Died Dec. 15, 1887 Aged  42 years, 2 months, 6 days. 

And according to a nifty tombstone birthday calculator on the internet, she was born October 9, 1845.

But the book Records of the People of the Town of Bethlehem has a slightly different story to tell. 

First, Mary is clearly identified in a 1869 listing of members of the First Reformed Church of Bethlehem, and I quote, James Mull Selkirk (and wife) Maria Selkirk (nee Wolfe) Mary Selkirk (their daughter) Mrs. Fredericks.  She is also listed as member in 1873. Going back some pages brings one to to an entry about baptisms at the church. There we have Mary and her brother John being baptized in 1848.  Her birthday is listed as September 21, 1845. Which is not what is on her tombstone, at least according to the calculator.  Her death is recorded in the listing for Elmwood Cemetery with the dates from her tombstone. Barney Frederick (or Fredericks) is only listed in the book as Mary's husband. 

I first find Barney in Guilderland in the 1850 US census when he is two years old living with his widowed mother Barbara who is just 21. 1855 and 1860 he is still in Guilderland, but in 1865 he turns up in Bethlehem in the household of Henry and Sarah Westfall as a servant. 

Mary Selkirk, clearly turns up in her parents household in the 1870 census in Bethlehem. In 1875, Mary and Barney are married and living with her parents in Bethlehem. Same in 1880. Then Mary dies in 1887 and I can't find anything else about Barney. (There is one promising death notice for a Barney in Ohio but probably not our guy) Where did he go? What did he do? No idea. 

Which brings me back to the tombstone with the miss-spelled Selkirck (and maybe the wrong age and birthday too).  What happened there?  Who actually purchased the stone? Barney or maybe Mary's parents? Didn't they notice the typo and couldn't they send it back? Was it already paid for? Were they just stuck with it to they put it up anyway.  I have questions!!!!