I think you've arrived at the most plausible explanation for the visible worm trails. Ive seen this type of damage appear on 17/18th furniture that has been refinished via mechanical means such as surface scraping/ sanding etc...Further, I have seen several American longrifles with English provenance that share this characteristic, as well as a number of other arms. An early English musket formerly in the Clay Bedford collection was heavily etched by worm action, so much so that the stock had to be conserved with resin fillers just to keep it in one piece. Bottom line is that this is a fairly common condition problem on early arms, particularly those coming from the UK or Northern Europe.
For what its worth, i like to current sideplate, you've documented the original form photographically, so i'd leave it as is...the replaced cock looks good and completes the character of the arm.
Bruce
