My background is running smaller engineering and manufacturing intensive
companies where we deal heavily in part numbers.

Regarding your database question, I would echo Jerry's answer. Anytime
that a field can be "messed with" by outsiders (in your case, the part
number) I would recommend not having it be the PK.

Regarding your company's number changes, it's behavior per your first
paragraph "For example Part ABC-100 might be changed to ABC-200 and the key
ABC-100 may be used for another part. violates "Part Numbering 101" and
basically makes the part numbers meaningless.

The description of behavior in your later paragraph "There is a white
envelope that measures 6” by 10” and the part-number is ABC-101. A switch is
made to a new supplier and part ABC-101 is replaced by an off-white envelop
that measures 6” by 10.1”. " is incomplete/ambiguous regarding what you are
actually doing in that case and so I can't comment on it.

Your company needs to start by defining what "part number" actually means to
it...that definition should have some enduring aspect, and then they should
follow that definition. The vague outline for this is that part number
includes a definition (which never fundamentally changes) that nails down
enough to fulfill a certain form fit and function. "Form, fit and function"
could be very loose or very tight, here are some examples:

Loose: Where the part number is a "sales number" that is exposed to your
customers. And the only enduring definition is that it can anything that
nails down attributes that are relevant to customers.

Medium: Nails down form, fit and function for manufacturing purposes.

Tight: Nails down every attribute, including vendor etc.