If you replace the 15 round base plate and spring with the 13 round base plate and spring you turn the 15 round mag into a 17 round mag!!!!! That's right 18 rounds in a Hellcat!!!!
If you replace the 15 round base plate and spring with the 13 round base plate and spring you now have a reliable 17 round mag! +1 in the Pipe . 18 rounds in a Hellcat!!!