In a race each horse shall carry a conspicuous saddlecloth number corresponding to his number on the official program. In the case of an entry, each horse making up the entry shall carry the same number with distinguishing letter. For example, 1, -1a, -1X. In the case of a field, the horse comprising the field shall carry an individual number, that is, 12, 13, 14, 15 and so on.
58 Pa. Code § 163.223