Verifying a Posi Differential unit?

[custom10nut]

I’m starting a search for a Posi Differential for my 70 C-10 (I’m currently running the original “One Wheeler” with 3:73 gears)
While I’m at it with the upgrade, I plan on installing a more Highway friendly Ring and pinion gear set.
Other than happening upon a unicorn truck, sitting in a yard with the Posi Rear option showing on the SPID,
How do I verify that I’m getting an actual Posi unit?
Are there casting numbers on the rotating assembly,
or are there numbers stamped on it?
In either case, where would these numbers be?
I know these are a lot of questions but I just don’t want to get “took” out of ignorance.
I’m hoping someone would have a picture of a Posi rear end with the cover off to show the gears.
Thanks in advance