Bob-weights have absolutely nothing to do with CG limits. The stabilty margin will shift with CG limits and the early mark Spitfire did have stable load conditions.
However all of that is completely irrelevant. CG shirts from consumption of consumables like oil and avgas. The NACA was well aware of all these characteristics and could do weight and balance.
The solution for the unacceptable and dangerous longitudinal instability of the Spitfire was bob-weights. These were added not because the NACA made a mistake in some half baked theory on weight and balance calculations. They were added by the RAE to correct a serious stability and control issue with the design.
