It's time to truly ENJOY my hobby again.
I'm from Illis but I've never heard of using Penna. Looks like young Jack was working on a unibrow there.
I've actually seen "Penna," as I grew up in West Virginia. I think it was on old Turnpike signage, but never noticed it on cards.
Nice find, this Jack Clark looks sharp! I grew up in the late 80's/early 90's so I dont remember him being young and slim. And I think I've only seen "Penna" used on old timey signage. Topps had some strange ways of abbreviating when they couldnt fit a city or state name on the back of the card. I had a Warren Spahn '63 Topps card that listed his home as "H'rts'ne, Okla." Like.. huh? Where is that? What is that?
I had the same thought as you about "Slim" Clark. I have to go check for the Spahn card now.