[SetBuilder]

I don't believe those signatures are secretarial. The experts could be wrong.

Mathewson's usual signature length is very small. If you look at all of his signed album pages and other items, the signature remains under 3" in length. Most signed checks have signatures measuring < 2.75". Mathewson had tiny handwriting.

The bookplate pasted on the front board of Won In The 9th measures around 5 inches, with the blank space for the signature nearly the entire 5 inches across.

If Mathewson decided to enlarge his signature for whatever reason, maybe to make it more bold, or to take up all of the 5 inches, his handwriting traits could have changed. Doubling the size of his signature may have caused this change, which manifest as some atypical traits. Could be the result of exaggerating the size of his normal handwriting.

Besides, why would he hire a secretary to sign the 500 bookplates? Was he so busy? That's kind of weird. Did ballplayers back in 1910 usually have secretaries?