That was indeed John Spencer, who is generally considered one of the lead actors of the series, along with Martin Sheen.
John and I waited tables together in NYC back in my acting days 20 years ago. My brief brush with that little corner of fame. I feel a bit protective of the reputation of my close personal friend JS. (Who wouldn't now know me from Adam and whom I haven't seen since 1980.)
I don't watch TV much (who has the time!) so when that face came up in the photo, I had no idea who it was. Neither did Steve Cohen. It's just one of those little details that we ran out of time to deal with before going to press.
Way off topic, but perhaps of interest, many years ago I baby sat for Martin Sheen. That is, I watched his kids, Emilio and Charlie. Martin was, way back then, a helluva nice guy (as was his wife), and I'm sure he still is.
I also graduated from NYU Film/TV grad school with a guy named Martin Scorsese, but that's another story.