USED CMMG FNH FiveseveN 10 Round Mag Extension Kit (Item # 10202)
I bought this just to try out, but found out that you need to sand and debur the magazine interface to get this extension to run reliably and I didn't want to mess with that.
The extender and spring were used for maybe two range outings, and look like new.
I paid $25 new, am selling used for $10!
Flat rate USPS shipping of $3
Merry Xmas and Happy New Year!