plate

[yukon 331]

took this of the under side of the deck is it a spacer.

[Kent 435]

Yes.  It is used to space the blade down to the same plane as the others.  Some of the new arbor shafts and housings are not as long as the old style.  

[yukon 331]

thanks kent.

[CarlH 419]

For some RBT decks a cast aluminum spacer ~1/4" thick is used.