Now I know why used parts are hard to find. 
Well, I found a 510 table and ordered the 520 rails from SS. Found a fence and ordered it as well. The guy I ordered the fence table from went on holidays and I had to wait 3 weeks to get the table. I knew about the carraige difference because I ordered a used carrage so it would clear the dust collector outlet and discovered that it was wider. Hence the hunt for the bigger table and better fence. Started to put things together and discovered another problem
(see pic) Anyone have a part number for this? Its part of the 520 fence. This slides along the rail.