Old Garnavillo Cemetery
Garnavillo, Clayton County, Iowa
Per Sharyl Ferrall it is located behind the Lutheran church, but the cemetery predates the church, and is not affiliated with the church.
I am not really sure who this is. There were two Heillers in Company D, 27th Iowa. This does not appear to be either one of them. Sharyl says the name was very clear on the tombstone. Henry Heiller is buried in Minneosta. William Heiller died in 1868 in Garnavillo, so it would appear that it would be him. But the name is not right. Either there was a John Heiller left off the roster, or the wrong name was put on the tombstone.
Sharyl's theory below is a possibility: William Heiller did have a son named John.
"Headstones Provided for Deceased Union Civil War Veterans, 1879-1903", image 679 shows that a military headstone was provided for John Heiller, Garnavillo cemetery, in Sept 1886 by WH & FS Gross. I can't read the next line because of the hole punch in the card but on the next page is an entry for a stone supplied by Gross Brothers of Lee, Mass. so I believe it is the same company. They probably made them & shipped by rail to wherever the cemetery was.
Very doubtful that it can be proved, but here is my take on the issue. There is no other explanation (I'm getting passionate about this now). If the DOD you have for William Heiller (09/03/1868) is correct, he most certainly was buried in the old Garnavillo cemetery. He may have had a crude gravestone at the time of burial, but nearly 20 years later his son John (b. 1851) finds out that he can obtain a marker for his father's grave from the US Government, so he orders the gravestone. Somehow the information is mixed up, and because John had ordered the stone .... Jno Heiller is inscribed instead of William Heiller.