I haven’t noticed the license plate thing, but since you mentioned the 555 telephone prefix, I thought I’d link to what Cecil had to say about it back in 1978.
What about 853 OKG? 
As a movie and tv show (dvd) collector I noticed that a lot of the vehicles that have the plate PCE are more likely for a police show or movie. Also I build models as a hobby and am looking for a site that has the correct plate #s for the vehicles, I want to be as authentic as possible any help would be appreciated.
Thanks
Kevin Lee Beard
A modeller, huh?
Wikipedia has a list of the current formats. For historical plates, your best bet is a Google image search for the desired state.
Ebay. There are people selling decals or paper printed plates with all sorts of TV/movie plates. Emergency, Adam 12, Vanishing Point (of course!).
For ones not available, try Acme License Maker. The fonts are not perfect, but you can get a passable looking plate. Not just recreating a movie plate, but you can make any combination you want. They have many old style plates - including CA black and CA blue. You have to print it out yourself. I use photo paper and an ink jet printer.
Also do an internet search. Sometime someone has a photo of the real plate that is good enough quality to download and print out.
My original MPC 1970 hemiCuda model has a plate that reads “426 PCE” as an homage to both the engine and Universal and their PCE plates. “1-Adam 12. No wants on 4-2-6 Paul Charles Edward.”
the License plates starting 000 - 499 PCE are all commercially owned plates Owners include Film and commercial makers for Cinema and Home TV ads, Due to the sequencing 211 PCE is owned by 20th Century Fox and MGM UA However Universal and Commercial Production companies own 213 PCE up to 400 PCE this constitutes the bulk of the motion picture industry.
Hope this answers your friends question
Never noticed the PCE thing, but pay attention and you’ll soon see 2GAT123 everywhere. That’s the 555 of license plates.
The only plate I’ve noticed being used more than once is THX 1138/THX 138.
I watch a lot of crime docs, and every car in the recreations on “On the Case with Paula Zahn” uses the same license number, 1GAT (some other numberals I don’t recall at the time).
That was Steve Martin’s licence plate in LA Story for one, it’s always stuck in my head.
Some observant person noticed the license plate NCC 1701 used in some movie, I’d be surprised if that was the only one.
I’m amazed how much Universal studios souvenir license plate OUTOFTIME looks like an actual Florida license plate. The first time I saw one I thought it was real.
I noticed PCE in several movies - most notably for The Big Lebowski:
But others include Agents of SHIELD:
Transformers:
Stranger Things: