Frackin'Stein

The Seventh Shirt...? It's Hethven Hert's: A Nonsense Leading to an Anagram

My sweetheart's sewn 
some seven shirts. 

The first she sew'd

is Blob-I-Dob's. 

The second sewn

is Seuss's Yert's. 

The third shirt sewn 

is Uncle Bob's. 

The fourth she sew'd 

is Fran De Boo's. 

The fifth shirt sewn 

is Gammer Gurt's. 

The sixth she sew'd...? 

I've got no clues. 

The seventh shirt...? 

It's Hethven Hert's.

For Blob-I-Dob see Delicious Nutritious Sayings. For Yert see Yertle the Turtle by Dr. Seuss. For Uncle Bob see Bob's your uncle. For Fran De Boo see Deliciou Nutritious Sayings. For Gammer Gurt see Gammer Gurton's Needle. There is at present no known source for Hethven Hert.