If you  have the schematic of the saw parts it should state what size the thumbnail is.  Take the other one to the hardware store and measure it.  You should be able to get a thumb screw there.  If you don't have the parts list or schematic call Delta and request one.  I did that to a table saw I bought and they e-mail it to me

 

Good luck

 

Dennis Goodue

The Vermont Scroller