Mr. Frog-- I did something today that I may regret. I stopped by my neighbor because I know he has a 150 lb PW, and we flipped it over. You'll never guess.
There is an "O" in the same spot, same size, stamped not quite as deep. My phone was dead so I didn't get a pic, but he told me to stop by this weekend and I'll scrub her up and get a shot. So, unless someone was flipping these things over and playing tic-tac-toe, the plot thickens.

Good grief, I'm laughing at this. ....
Fantastic information! What is the vintage of the "Type O" anvil?

now we know they were playing tic tac toe in the foundry

Just on another way-out-there crazy idea.... Looked up the game history of tic-tac-toe.

Wiki says:
....The different names of the game are more recent. The first print reference to "Noughts and crosses", the British name, appeared in 1864. In his novel "Can You Forgive Her", 1864, Anthony Trollope refers to a clerk playing "tit-tat-toe"

If it made it to print by that time of 1864, it had to be getting popular well before that.
Just throwing another crazy idea out there....

Same vintage as all the rest.