the 4.1's are two seperate pieces consisting of the crown-wheel and pinion, they are attached to the actual "centre" and pinion carrier via bolts.

if you were to get a full spool. you would simply unbolt the "4.1's" and bolt them up to the full spool/pinion carrier.

if you have never touched pinion carrier before, i wouldnt recommend touching one as there is a lot of finesse that goes into aligning a pinion with a crown-wheel but thats a whole other bag of cookies and should be left to the experts.