The sander you have is a 503; the 504 had no provision for dust collection.
On the subject of Porter-Cable sanders, meet the family.
This is one of every worm drive belt sanders Porter-Cable made.
Left to right: 500,503,T-4,T-33,T-3,BB-10,B-10,B-4,B-5,A-3,A-2. The A-3 became the 504, and it...