If I remember right there are actually three different slide action assemblies ranging from 6 5/8 inches to 7 5/8 inches and to 8 5/8 inches. There are also differences as to whether they have a single or dual slide bars.
Recommend you measure the length of the assembly to make sure which one you need.
Mine has the dual bars and is 7 5/8". The new forend is for 6 5/8". The problem is that I can only find the one I bought (16322). I've searched plenty of vendors and Mossberg's website too and that is the only synthetic forend anyone lists.
The main reason for wanting a synthetic forend was to match the 510 Mini stock (which I can't find in stock either). But I guess I will have to go to plan B and just leave the wood forend on and get another wood stock so I can cut it down to fit my petite wife.
The new forend length is between my older 500E and my 410 Shockwave.
I have already checked all those sources. The only synthetic forend listed, let alone available is the one I bought. And I have not found any extended slide assembly nuts or adapters for the 410. I appreciate the help though.
And unless I can find a 510 Mini stock anywhere in stock, then it will definitely have to be Plan B anyway.
And it seems I have an odd ball. My 500 has the 7 5/8" slide tube assembly with dual bars and all I can find that length online is the single bar version. The search goes on for the smaller stock before I decide what to do about the forend.
I know this is an older thread but I did finally get my hands on a 510 stock with the extension. So at least I can fit the shotgun to my wife now. I'm still trying to figure out what to do about the fore end.