As far as I know, playfields used to have a top veneer of maple, based on the recent pinball news story about churchill seems that at some point, not sure when, they switched to Baltic Birch, which, in general, is not as hard as maple. I've used Maple on one of my custom games and Birch on the other, and there is a clear difference and in how they hold up, especially with no clear coat. The birch does come in 9ply, which should be stiffer and less prone to warping due to weight of mechs, etc., I've never be able to find maple with more than 5 plys, and sounds like the old playfields were all custom plywood. I suspect a big price difference (and the availability) between maple and birch is a factor in all of this.