Just before heading off to work over the weekend, I found this interesting article by Tsuchiyama on Nobunaga's hand signature (Nobunaga no kao wa naze bimyo ni henka no ka). The article stated that in 1567, right after the conquest of Mino, Nobunaga's hand signature was now formal. However, his signature changed every year and on pages 4-5 displayed his handwriting and how it evolved.
Here is the link to the article. The article is in Japanese.
Nobunaga no tame!