Module Code.  The most familiar designator of a module's series, such as "A" for Aerie of the Slave Lords, "B" for Basic, etc.  These codes were used through the mid-1990's, when TSR decided they were becoming too cumbersome.  A complete list of module codes, and their associated meanings, can be found here.  The Module Code, when present, is found on the upper left corner of the front cover, in bold.

ISBN.  International Standard Book Number, established for all printed books since the 1960's.  The ISBN is associated with a machine-readable barcode, which often appears in close proximity to the actual number.

Bar Code.  Sometimes misconstrued as the "UPC" code (which is something else entirely), this barcode actually reflects the ISBN of the item.

Piece Code.  A string of digits assigned by TSR.  The last four digits of this code are usually identical to the Stock Number.  The Piece Code will be found on the back cover of the item, if present at all.

Product Number.  An even longer string of digits assigned by TSR.  The suggested retail price of the item is often disguised within this number, at the end.  The Product Number will be found on the back cover of the item, if present at all.

Stock Number.  For pre-1990 D&D products, this is usually a number within the range of 1000 to 9999.  The complete range is actually 0001 to 99999, with a few oddball codes (some beginning with a letter) thrown in for good measure.  The Stock Number roughly corresponds to when it was released, so this can be helpful in product research.  A reasonably complete list of TSR Stock Numbers can be found here.  The Stock Number is almost always found on the front cover of the item, as well as buried in the Piece Code, if that's present.