If your Model 66 is like my Model 66, and yours looks similiar, that post is an integral part of the barrel.
That red plastic insert is 70's (maybe 60's) technology. I would not recommend knocking it out, unless you are absolutely sure that you have a drop in (and glue in) replacement.
These older pistols have their limitations. No place to attach the turbo, no power port for a death ray.