What you already have. It's the Product Name: 3-Way Wall Switch Module - WS4777
Description: For control of any light that is operated by two or more switches. Upgrades your existing standard wall switches.
Details: For remote and local X10 control of two or more switches that operate the same light, upgrade your existing switches with the 3-Way Wall Switch Set. Each light requires a master switch, plus one slave switch for each of the additional control points of the same light. Push button ON/OFF control of incandescent lights up to 500W. Responds to ON/OFF, DIM/BRIGHT, and ALL LIGHTS ON/ALL UNITS OFF by X10 signal. Features: Rated at 500 watts (incandescent). Dimmable through X10 controllers.
Since yours is five years old, unless the main switch has somehow failed, it should work. Since you have 3.305 look in the old module section and see if the WS4777 is still there. If it is, replace that with the module you how have. 3.305 is known for not working well with .ahx files from earlier versions.