It's a wild guess, but could it stand for some kind Yorkshire regiment unit in Korea? Y 01-k -51 is handwritten in my late norwegian fathers driver lisence from 1984, as where he got his lisence the first time. It's a puzzle, but he wore this pin, and may be these two also have something to do with Korea? -- Med venlig hilsen Bjørkmann
Saku, you probably have discovered, is a place in Japan. That looks like a commemorative medal, coin or badge. Was your father an active Christian?
I only mention it as I've seen lots of commemorative tokens for church conferences, camps, meetings and synods etc. The 'Y' patch looks naggingly familiar, but I can't place it.
5th Infantry Division. Black circle & white Y used on vehicles & signs in WW2 . Postwar it was used on uniforms too. There is a thread about it on here.
It might not refer to a place. It is quite a common personal name in Finland and also applies to many company names in countries from Estonia to Sweden. The Saku company began brewing in Estonia in 1820 and is now an international drinks company
I might've missed it elsewhere but what's the Yorkshire connection? It would be helpful to see an image of the handwritten note on the licence. To me your transcription "Y 01-k -51" looks like it's could be read as Yorkshire?
The Yorkshire connection is another trail I have been researching, not shown here, badges, and a foreign dog tag etc.