The introduction of a restoring force technique has allowed the internal stress within kiln-dried lumber to be assessed, without requiring a modulus. However, an analytical model based on the elastic beam theory was only valid within a flexural range and only internal stress in the direction of the width was assumed to exist. Within this work, the model was extended to lumber, which could also contain some other internal stress components, by testing 30-mm-thick and 100-mm- to 130-mm-wide flat-sawn rubberwood boards. The improved model successfully separated the effects of other internal stress components in terms of a remnant force. Also, a finite element analysis was employed to validate the internal stress behavior. With little impact from other internal stress components, the finite element model, which used the released strain and Young’s modulus in the tangential direction, successfully simulated the restoring force profile for an entire half-split length, including the relatively short half-split length outside the flexural range. But the finite element model failed to perfectly capture the restoring force behavior in the presence of other internal stress components. Future work is required to fully investigate the internal stress in all three main orthogonal directions.