So what is not true about what I said?
From what I read, the OP was asking about the format of a system key.
If you reread my post, I prefaced my comment with regard to software keys only, and there is no difference in the format of a software system key for a P25 system vs a legacy SN/SZ system.
you said:
"as far as software system keys go, there is no difference in how they work."
That is not a correct statement.
There are great differences in what Motorola CPS of different product lines allows one to do. In case of Kenwood an EF Johnson keys for P25, other things come into play.
Software keys do NOT allow you to program Smartzone OmniLink or ISSI roaming, nor they will not work on newer products like APX without a special flash option.
So there are key differences in how they work. OP was asking about Project 25 system keys, presumably for Motorola Astro 25 subscriber radios.
Kenwood P25 keys are totally different, as are EF Johnson, even for Motorola Astro 25 systems. Kenwood, for instance, keys are "married" to a serialized copy of KPG, so only authorized persons can program subscriber radios, and connect to an authentication server.
So your analogy isn't really relevant.