What is this key used for? Any info good.  *****the last mark on both
sides of the key is not and "l"  .   that was close.  it is actually a
stright line. the line is smaller on the do not side and appears a bit
wider on the P side?

Brass Key- first side

DO NOT
DUPLICATE
l

OPPOSITE, SIDE TWO:

P7  Pl

It could be a key to anything. There is no way we can tell you what
the key fits, but the fact someone thought to have the warning stamped
on it means they want it back.

The "Do Not Duplicate" simply means that when the key was cut, the
purchaser asked for the "Do Not Duplicate" be stamped on it. This
makes it a bit more difficult (but still possible) to get a copy made.

Engraved Warnings.
http://professional.schlage.com/app_solutions_key.asp
..."Often you may see the warning "Do Not Duplicate" stamped on a key.
While there are laws prohibiting the duplication of these keys, they
are extremely difficult to enforce. Locksmiths and retail stores
routinely duplicate these keys with no questions asked..."

Typically Safety Deposit Boxes have keys that say Do Not Duplicate.

Bank Bags that ae used to hold cash, in order for businesses to make
their Cash bank Deposits, thsoe locking Bank Bags will also have keys
that are marked Do Not Duplicate.

Many keys to businesses will be Marked Do Not Duplicate. A Restaurant
Owner needs to give a key to the cooks and cleaning staff, the
restaurant owner will aks for keys marked Do not Duplicate.

These are a few examples.

Why not post a pic of it and maybe it could be narrowed down abit (ie
Safe Deposit vs Bank Door look alot different).

It could be anything(like the first guy posted)  the code probably
corresponds to a secific lock (i.e. the same number will be on a
lock).  The code is also used in case the key is missing, if you know
the code, your facilities manager may be able to make a new one from
the code.  The code on the key also makes it easier to key another
lock to that specific key.