My guess is Japan, but my Japanesse is none existant, and I've never had much success dating these...
You got that right. Aluminum, 1.5g. As the war went on, and aluminum became scarcer, the weight was reduced to 1.2g (Y#61a) and ultimately to 1g (Y#61b) in 1943. The diameter remained constant, so the three versions are sometimes referred to as "thick", "medium", and "thin" planchets.
