Daniel , according to the  Steve Blanchard Splitdorf site , the DU7 was developed in late 1927 , and introduced early 1928 . The short frame 1928 CG and CGP models were in production from September/October 1927 until March 1928  , when the 101 was introduced . With that in mind  , the change from DU5 to DU7 was probably introduced as and when stocks of the DU5 ran out .
Du5 is fine .