I smell a GG&G lawyer. I suspect GG&G is hesitant to say it's compatible because of the wording in the ATF determination. The ATF specifically said the Mossberg Shockwave is legal, based on the sample firearm that Mossberg submitted to them for review. The ATF also said that they could change their minds if the grip configuration changed, if the construction materials changed, if the color of the sky changed, etc.
When it comes to recent firearms that hit the retail market, the Shockwave was one of the most mysterious in terms of "How is this thing legal????" I wouldn't be at all surprised that the good folks at GG&G said "The grip is what makes the gun legal. We ain't touching that grip. Leave it alone."
That's all speculation on my part.
What ain't speculation: The 590 Shockwave is built on a standard 590 receiver. See pic below. An antique Mossberg 500 buttstock fit my 590 Shockwave perfectly.
Mossberg did not tool up new receiver blanks, etc for their Shockwave model. In fact, it's called the "590 Shockwave" because that's the name of the company that manufactures the Raptor grip. Shockwave has been making the Raptor grip for years, and lots of people have purchased them separately and put them on their standard 500s and 590s.
I took the Raptor grip off my Shockwave and replaced it with one I made by hand from wood using an antique 500 full buttstock. They all use the same standard dimensions and are all interchangeable (which makes it super easy to commit a felony, by the way).
One of the great things about the Shockwave is that it's a very economical way to build your NFA short barreled shotgun. You can buy a Shockwave cheaper than you can build one (using a 14" barrel kit). So a lot of people are buying a Shockwave and immediately applying for an NFA tax stamp so they can put a full shoulder stock on it.
I'm one of those people. I just bought a second Shockwave last week so I could SBS it. I plan on putting a Magpull SGA buttstock on it once my stamp arrives.
Keep in mind, the 14" configuration with a bird's head grip has always been legal, so long as the gun is a minimum of 26" over all. It was legal before the NFA passed in 1934, and it was legal after the NFA went into effect. Nothing changed. Mossberg simply never made one from the factory in this configuration. Other companies, such as Asylum Weaponry, purchased 590 receivers, 14" barrels and Shockwave Raptor grips, then assembled them under their own brand and sold them as the "Gatekeeper" firearm. They wouldn't have been able to do that if they needed Mossberg to manufacture custom spec receivers just for the Raptor grip. It would have been cost prohibitive. And Shockwave Technologies would have never been a commercial success if they made a Raptor grip that didn't fit a standard receiver.
I'd bet my next paycheck that the GGG-1429 will fit, because any standard Mossberg 500 or 590 buttstock fits. If the GGG-1429 has any issues at all, it would be that you might need a longer grip screw. I needed a longer grip screw for my wooden Raptor grip because of the depth that the wood buttstock's screw orifice was drilled out. Not a big deal. The allen bolt you see in the top pic is the original bolt that came with the plastic Raptor. It was too short for my wooden version of the Raptor. So I just used the original buttstock screw that came with the wood stock. That's a 36 year old buttstock and screw from a Mossberg 500. And it fits just fine. The screw is ugly and chewed up, but it fits.
Oh, and if the ATF is reading this: The wood buttstock below was cut down to a rough raptor grip shape BEFORE I ever attached it to my firearm, so you can kindly kick rocks. No felonies were committed during the modification of this firearm. Nothing to see here, please move along.