Not quite, you find a repro drum and have the spine from a box mag spotted to the back. Then it just slides up in place. No need to refit uppers and lowers...
I had three of those through time, a Royal Typewriter 1918A2, then a Marlin Rockwell 1918 with the walnut stock and no over strikes, finally a NESA like yours...forget where my number fell in relation to yours...