The OEM belt configuration for the Vermeer 503 I is:
Part# 21350450
Part# 21001540
Part# 25049505-C
Part# 25050605
Part# 25050630
Part# 25050618
Part# 25069770
Part# 25047028
Part# 25048602
Part# 25048601
Part# 25047076
Part# 25051005