If it is an OM box,, it definately has value. Especially if it has another label stuck over the original printed area stating it's a NM. Details & pics help a bunch.
Bacically,, the OM boxes were 2 piece, black, yellow, type. When the NM's came out,, to avoid waste,, NM labels were added to OM boxes & NM guns were shipped out. When the supply of OM boxes were exausted,, they all became the red, hindged lid type used for NM's for several years.
So,, go back,, check it out,, and if the box is 2 piece,, and does NOT have a NM label,, it has value as an OM box. If it does have a stick on label signifying it's a NM,, and it's a 2 piece box,, then again,, it has value. After that,, condition and markings come into play. Is it for a 7-1/2 like you think,, or is it for a 4-5/8,, or something else all together? Next,, inside,, does it have the brown wax paper, warranty card, & which model instruction manuel? ALL of these things contribute to value. It can be as low as $50 for it to as much as $150-$175 or so,, depending. The devil is in the details.