Luke, it will fit but Adding a/this Stock will change the classification from a "Firearm" to a SBS "Short Barreled Shotgun" and you will need to pay a $200 Tax Stamp and register it as an NFA Firearm. Details here : https://dailycaller.com/2017/08/03/navigating-nfa-rules-on-shotguns/

Luke, Illegal by federal law to install a shotgun shoulder stock on a 590 shockwave (Felony charge). $200 tax stamp, registration with the ATF all needed before you could make a shockwave an SBR. Will work if you want to go through expense and waiting period.

While it will fit the shockwave it will make it a illegal Short barrel shotgun without the proper NFA paperwork and tax stamp and could get you a long stretch of jail time. You would need the paperwork and tax stamp prior to installing the stock