Most, maybe all older 870s say WM on the receiver. Possible exception, the lower cost "Sportsman" that precursed the Express, and was available for a few years.
Depending on vintage, there may be a "Police Cylinder" marking for choke on the bbl. This was a few thousandths tighter than pure cylinder, like skeet choke. Otherwise, look for either Cylinder or IC marks.
Unless this is in pristine condition, $259 in a bit high for a turnin.I'd want to drop the trigger group and look for grunge and rust there and in the receiver before laying money on the counter. If it is clean and tight, maybe $259 is not a ripoff.
HTH...