Pitstop.
At this point, we must assume that everybody in the world knows Penny’s last name except the people watching the show.
Good idea, but in the context of the show, the most likely scientist is Steven Hawking. It’s plausible that the guys all know it, but have simply never made the connection.
Another good candidate would be “Tyson”. Penny visits Leonard at work - probably for another bout of kinky laboratory sex - and runs into Neil. “Neil! I didn’t know you were in town! Guys, this is my cousin Neil from New York!” Cue awkward moment of racist embarrassment when guys are surprised that Penny’s family is bi-racial. Cue awkward moment of intellectual embarrassment when Penny is surprised that her dopey cousin Neil who she used to beat up is a famous astrophysicist.
Harkening back to an earlier episode, my suggestion is Blossom.
Also harkening back to an earlier Mayim Bialik role.
Whoa!
No, she never beat up her cousin Neal. He was always a badass.
Oh, and I also don’t think that the writers originally intended for her last name to be a deliberate mystery. My guess is that they just never thought of one because it never had reason to come up in an episode, and that it wasn’t a big deal until some fans noticed that it wasn’t known and started speculating. Now, of course, the writers are taking that and running with it, so if they ever reveal it now, it’ll be a big deal.
Nooooooo kidding! Duh! I sure feel like a real dummy! :smack: 
Howard searched online for Penny’s Serial Ape-ist scene on the day he met her. Although this wasn’t revealed until the start of season 7.
He then showed it to the others. So Howard knew her name first, of the guys (probably), followed by the others. Assuming Sheldon was paying attention.
If you didn’t actually think that, why did you say it in your earlier post?
Too bad there’s no rank of ‘Penny’ in the US military.
Penny R. Caide
Penny Pasta. (That’s the Nebraska spelling.)
Thematically, the only possible last name would be Bershad.
Post 2.
Her name is Penny Schrödinger. Or not.
Bugger. :mad:
What makes you think that’s strange? Farmers can read Greek mythology, and classical names were very common in parts of the USA at one time.
Oh, that’s been covered.
Well, I like the Everett idea; that’s clever.
Well, Sheldon was with Penny in an ER. He didn’t ask for her insurance card or her license. He could have just written Penny and left last name blank. He could have written “Penny WiFiStealer”.
Possibly, he was told along the way somewhere that he was beyond the point where social convention dictates you cannot ask a friend their last name.