Here's some trivia - who was the last Giants manager who ever played for a NPB team OTHER than the Giants? The answer is Hall Of Famer Haruyasu Nakajima, who managed them in two separate stints in the 1940's (1943, then the latter part of 1946 and the first part of 1947). Here's one of Nakajima's cards from the 2000 BBM 20th Century Best 9 set (#325):
Nakajima was player-manager with the Giants and later went on to play and manage for the Whales. The only other manager of the Giants who played for another team was Hideo Fujimoto, who managed the team between Nakajima's stints. Fujimoto played one year for Chunichi in the late 1940's. Note that none of these guys played for a team other than the Giants BEFORE they were Giants manager, only after. There has never been a Giants manager who played for any professional team other than the Giants prior to becoming Giants manager. In fact, the only guy who has managed the Giants without playing for them was the Giant's first manager, Sadayoshi Fujimoto.
One other neat thing about Nakajima - he was the first Giant to wear the number "3". He wore until after the 1942 season, switching to "30" when he became manager. In 1946, Shigeru Chiba took the number and kept it until 1956, when he retired (Actually, he wore it in 1957 as well as a coach). In 1958, the number was taken by Shigeo Nagashima, the last Giant to wear it. There's something symmetrical in three Hall Of Famers wearing the number "3". (What would make this perfect was if no one other than these three had worn the number, but someone named Mikio Tamura wore it for the 1944 season.)
I'd like to thank Japan Baseball Daily, Jim Albright's list of Japanese managers by team and the Player Number History web site for all their help with this post.