NLA. depending on caliber get a blank with extension from benchmark (they no longer sell extension by themselves)
The other alternative is have extension cut off and treaded from your existing barrel. Both options need a smith to do the chambering. Depending on what caliber your rifle is in will dictate your choice. Md Charlton won’t be able to provide you any parts or services.
If I remember correctly some of the short action calibers .243 maybe 22-250 have a different bolt head and won’t work with the benchmark extension.