Nitro:
Getting back to your original question, it was fairly common in the late 19th and early 20th centuries to "plug" flaws in an otherwise good piece of stock wood - generally knots. To hide the plug, a panel of chequering was often added, per the gun you reference. This is probably what you have seen. If so, the stock is probably fine and strong.
On the other hand, many Parkers and other US guns stocked in black walnut often split their stock heads and a cross-dowel was installed to fix the problem years ago. This does indicate a repair for a wood failure.
I'd bet the Francotte you've seen is fine, however.
Best, Kensal
