IIRC, Mark does all of his fill ins from some studios just South of the border in NH (Icestation EIB, I think he calls it) and the networked transmission goes out through a station/studio in NYC.
(Last time I went to NYC on business, I told the local sales rep squiring me around the job sites...old dockside warehouses..that there was Good News/Bad News about the place: Good News that I was checked out on the M-60, Bad News that I didn’t have one along.)