Could have been setup for stock stroke with Yamaha domed pistons, or +4mm or +5mm stroker with kawi pistons and no spacer plate or Yamaha pistons, using a stroker plate, or ???? who knows. Are there any markings on the top of the domes? Usually there is a cc size engraved or stamped in. they almost look like flat-tops (kawi style) in your pic, but hard to tell.
My second cylinder is a similar height, has a spacer plate, was using kawi pistons and has a 7 port configuration like a "Lamey" cylinder...
There seems to be many mysteries to these old billet relics...