I am in communication with G.F. Heizer.I pointed out to him that the Thomas H. Shearer who married Maria A. Heizer could not be the daughter of Valentin Heizer as the marriage took place in Marion Co., IN 21 Sep 1871 (Indiana Marriage Collection 1800-1941 found on Ancestry.com).I don't know his source of the marriage he mentioned but I know G.F. took Heizer information from the book titled The Heizer Family, American Pioneers.There are proven errors in this book, which I won't go into now, but all information needs to be checked out.
"It is my opinion" that Mary/Marie went to VA with her family and married Peter Grass.She is near the same age as Peter Grass and it just makes sense to me for other reasons.
My ancestor, John Heizer married Elizabeth Mowry.Her uncle, Henry Mowry, in his will, mentions a 152 1/2 acre plantation that he purchased from Peter Grass.Also, on the list of names to do road work from Staunton to Christian Creek (Augusta Co., VA) is Peter Grass, Henry and Peter Moura (Mowry) and William Burk.
John Heizer bought 120 acres on a branch of Christian Creek, Augusta Co., VA from his uncle-in-law, Henry Mowry.
This all adds up that Grass, Heizer, Mowry and Burk families lived in the same area and knew each other.Not a lot to go on but does encourage me in the direction to believe that it was Mary/ Marie Heizer who was the wife of Peter Grass.