Hi,
Am I misunderstanding something? If g.grandfather was officially J. W. Burgdorf, then surely he would be known as Wilhelm in Germany. If he named his only son Wilhelm then it looks like gt. grandfather Julius Wilhelm was the oldest son of a Wilhelm Burgdorf.
The problem with ages on death records often occurs when a grandchild or an in-law gives the information - maybe if you double checked the months on the two records you may find J.W. had just passed a birth date on one of them.
Best wishes,
Rena in England.