On the North side of Terminal 5A are four stands, 501, 502, 503, and 505. Why is there no 504?
Stand numbers at Heathrow correspond to the terminal and gate numbers. The first digit of the stand number is '5' for all terminal 5 stands, and the last 2 digits correspond to the gate number. So, the missing stands 504 and 510 would correspond to gates A4 and A10 respectively.
According to unverified (by me) reports on flyertalk.com, Gates A4 and A10 are exclusively "bus gates" where passengers board a bus rather than accessing via a jet bridge. So there is no corresponding stand next to the gate.