Some years back I think I posted here about a Dia-branded Mosrite bass copy a friend owned back in the '70s. It was suggested at the time that it may have had a connection to Aria (I.e., Dia = short for Diamond). The other day I was in my favorite used instrument store and saw two Japanese hollowbody electrics, one with a common Matsumoku brand and other nearly identnical guitar labeled Dia. I have pics but can't post them from my phone. I flipped the guitar over and, sure enough, it had the Matsumoku neck plate. I think that's enough evidence to add Dia to the list of Matsumoku brands in the Japanese manufacturers/brands sticky.
Scott