The Fender 9050L flats fitted as standard are very similar to chromes in feel and sonically, both being stainless steel flats — An unverified but often repeated rumour is that D'Addario make Fender's Strings. Either way it is a small compromise, and an easy one to remedy, but the man himself has mentioned on talkbass that he uses LaBella 760F-MUS on one of his JMJ mustangs and Medium Scale D'addario XL's (EXL160M) on another.
All three of my JMJs are currently wearing ECB81Ms and I have a few spare sets just in case I outlive them