How to determine metal action slide tube assembly size?

Can't seem to find anything outright telling me what size I need. I got a 500 from before 2020, and got an adaptive tactical forend. Says I need that part to install, the problem is I have two sizes to choose from but nothing telling me outright which one I need. Just tells me to match the specifics of the firearm. I have a 28" Mossberg 500 with a 5 shell capacity tube. If more info is needed please let me know, really trying to get this figured out.