I've been doing a bit of research on this as well. I'd like to replace my Maverick 88 12 ga 18.5" with a Mossberg 500 action slide assembly.
From
https://www.havlinsales.com/default.htm Attached FileComparison of action slide lengths: top 7-5/8”, bottom 6-1/2”
Measure from top of threads to bottom of collar that holds the action bars. This measurement is critical to fit when ordering forearms or stock sets that include a forearm. For the most part, 20-year and older sporting shotguns and new tactical shotguns use the 7-5/8” slide.
New sporting models have used the 6-1/2” slide for almost 20 years. There are exceptions, however. To be sure, measure!
NOTE: Since 2006, most new 500®, 535® and 835® sporting shotguns with synthetic forearms may not use an action slide tube at all; rather, the pump handle will have two integral bars pinned into the handle itself. If you have one of these, you cannot use a forearm that was designed to go over an action slide tube without first buying the
correct length tube and action slide tube nut.I see Havlin sells both length action slides (both oos) but lists only one Slide Nut for $14. I, too, thought there were two different length nuts.
Now, most everyone else - including Mossberg lists two different nuts.
Mossberg Part No SKU 13488 for 6-3/4" Slide Assembly
Mossberg Part No SKU 3408 for 7-3/4" Slide Assembly (oos)
Most likely - you have the 7-3/4" Slide Assembly - which means you need the Part #3408.. here it is from Midway (oos)
https://www.midwayusa.com/product/1005521465?pid=331117