LD1,
Rigby maintained a number of ledgers to manage their production. Included were ledgers that tracked barrels, actions, orders, serial numbers/delivery, etc. Generally, Rigby rifles and shotguns had a serial number, a barrel number(s), an action number and an "R&B" action patent use number if the Rigby & Bissell vertical bolt (rising bite) action was used.

My guess is that's the Rigby action number. If you can post additional information, I may be able to expand on what I said above.