Guessing it was some genius level deck-shuffling in the marketing department to brand as much of the line with the sporty look/feel of the fastback versions.

Then, S-55 came BACK in '66-7 (!) as the sports option for the Monterrey/Park Lane, again speculating here, but L-M was leaning into luxury vs. performance with the Park Lane moniker for the top-range.

I have a pic (somewhere) of dad's 1966 Merc convertible in the Diamond Blue over Blue, with the S-55 package, but in the Monterey trim and not the Park Lane (as depicted in the AMT kit.) That car marked one of the first times ever for a very young me sitting in the front seat - Mom came to pick me up from school in a driving rain, and I marveled at the shifter on the floor, first one I had ever seen! I also recall hopping in the back seat in the summer, wearing shorts, and that heavy-gauge vinly being about as hot as the sun itself.

Not long after delivery, thieves slashed the convertible top and well to get to the spare, IIRC, a fairly exotic tire for a Ford (possibly a wide oval.) In hindsight, I suspect the combined effort of two ne'er do well neighbors, one of whom was a noted short track racer and chop-shop operator. Many years later, he confessed to snatching a 1965 Lorenzen #28 Galaxie from behind the Ford X-Garage and rapidly parting it out. Charlie Gray later confirmed that the car had indeed been stolen, and it was quite a kerfluffle at the time. That all this happened in bucolic west Dearborn still seems rather incredible to me.

Said theif eventually was caught for other crimes, did a stretch at Jackson, and turned his life around. Ironically, his car number was...wait for it! ... #28.