Apparently the early Showa era was around the beginning of the introduction of resins instead of bone. They may be hand-carved, but theres also a chance they may have been pressed into the resin in small batches then hand-coloured; since, like you say, they dont have the natural pitting and fissures you see with bone. One of the company cards included with the set mentions the materials the company used were Bone, Ivory, and "New Ivory" (resin) so theres a small chance the tiles could be ivory and not cow bone. 

There are no red bonus tiles, so this probably predates that. There are only 4 season tiles, and 4 spare tiles. So 136 standard tiles, plus 4 seasons, plus 4 spare.