Michael, since there are always model changes, I would first confirm with Mossberg with the UPC/Model number if this is pre-drilled or not at (800) 363-3555 | 8:00am-4:30pm Eastern. Mossberg can likely answer all your questions in one fell swoop. Cheers and stay safe!

Another possible saddle mount
brownells.com/optics-mounting/rings-mounts-amp-bases/shotgun-bases/mossberg-500-12-gauge-saddle-mount-sku955000142-38855-62008.aspx

Mossberg makes a barrel conversion kit that they say fits all mossberg 500 20 Guage opticsplanet.com/mossberg-92010-500-barrel-20-gauge-24-rifled-cantilever-scope-mount-blued.html
or as randy said you can have a gun Smith tap and mount a rail.
Or B square also makes a saddle mount that fits most mossberg
I'd contact them to make sure this will fit yours
brownells.com/optics-mounting/rings-mounts-amp-bases/shotgun-bases/shotgun-saddle-mounts-prod1779.aspx

a good gunsmith can drill and tap it for a rail with no problems