Sorry to take such a long time getting back to Cynthia's Mystery Squash!
On the day that I read your post, Cynthia, about the Mystery Squash... I happened to look down at my Hollyhocks growing right outside my front door.
Could it be??? Hollyhocks???
The leaves of your Mystery Squash sure look like the leaves on my Hollyhocks!
But I don't know.... so my best guess... is that your Mystery Squash is a Hollyhock plant! Guess it won't be long before we learn the truth... Can't wait!!!
More info on Hollyhocks can be found here.