You have to read all the rules --
A pocket pistol is a small frame, fixed sight, pre-1900 design
revolver having a barrel length of four inches or less. Pocket
pistols must be .31 caliber or larger. Model “P” Colts and
clones and revolvers with swing out cylinders are specifically
not allowed regardless of caliber, frame size, or barrel length.
Pocket pistols may not be used as or converted to main match
revolvers.
The following additional firearms have been approved for use:
• Small frame Model P revolvers such as the Cimarron Firearms
Lightning, Uberti Stallion, and Ruger Single Six .32 H & R
Magnum.
The reason they specifically mention ".31" is to include the Colt 49 and Remington .31 C&Bs as there are no .31 cartridge guns. The rule then prohibits using or converting a legal pocket pistol into a main match revolver. The only small frame main match pistols allowed are the Model P juniors and the Ruger .32.