I know for a fact and from experience that the 46FX, AX and 50SX all have the same muffler bolt pattern.

I've never had a 55 but someone on here stated that the 55AX uses the 46AX Power Box muffler

And on the Tower site it says they are the same
http://www3.towerhobbies.com/cgi-bin...?&I=LXNXD9&P=Z