Well the details in that article suck..
It doesn't say the length of the original barrel.
So it's synthetic and has a 18" second barrel.
I scrounged this up.
O.F. Mossberg & Sons, Inc. - Firearms, Shotguns, Rifles, Accessories, and Precision Machining
Looks like there are a bunch of models.
52262 Mossberg 500 12M/20MC(IF) CAMO MOBU
52263 Mossberg 500 12M/20MC(IF)CAMO RTHWG
52132 Mossberg 500 BANTAM 12M/24MC VR
56420 Mossberg 500 12M/28AC SYN PRK PRTD
50120 Mossberg 500 FIELD 12M/28MC VR
54244 Mossberg 500 SLUG 12M/24RB RS PRTD
54264 Mossberg 500 COMBO 12M/28MC&24RB RS
54169 Mossberg 500CG CMB 12M/28AC&18CB
50577 Mossberg 500 12M/20CB BS SYN 8SHT B
50579 Mossberg SPEC 12M/20CB SYN&PG 8SHT
50411 Mossberg SPEC 12M/18.5CB SYN & PG
50580 Mossberg CRUISER 12M/20CB BS 8SHT
Looks to me from the website it might be a version of the
54169 Mossberg 500CG CMB 12M/28AC&18CB but it doesn't have the pistol grip does it?
It's the only one listed with a 2nd 18" barrel
The trim doesn't seem quite right (trigger group and furniture)
It might be a model from a previous year?