Quoted from JacksonsInLiss:A huge mystery I have is that the machine has been converted to Metal-Rails and the backbox is tapered??? In the two years that I have this machine I have not been able to find any explanation for this so if you have any ideas then please let me know
Maybe it was an attempt to (cheaply) modernize the game back when woodrails were becoming less popular to avoid paying for a brand new game.