Some CA off-list lower dealers may not want to transfer it with a fixed magazine. While legal to the letter/spirit of the law (i.e. conforming to the converse of the sec 978.20 detachable mag definition), it could be more dealer grief than a bare receiver during any DOJ inspection.
Just go take the rifle apart - take with/ship to you the upper and other parts, they're non-gun and not controlled. Remove the pistol grip and complete telestock assembly too. I recommend taking off the whole telestock including the extension tube, not just the actual physical stock itself.
Watch for and save the small pins & springs that fall/fly out - likely candidates are selector detent pin and spring, takedown pin detent and spring, and buffer detent pin and spring. Rest of parts should stay in, IIRC.
Then, have your dad's FFL transfer the lower to your CA FFL that handles off-list lowers.
When you reassemble the rifle inside CA, just remember to FIRST securely attach the 10rd or less fixed magazine - before reinstalling the pistol grip and the telestock and flash-hider equipped upper.
Congrats - those S&W carbines are nice, tight and well-finished. I handled a couple at the SHOT Show and they were great. It's nice to see S&W get into these.
Bill Wiese
San Jose