I was able to get 61K on a recent LHR>LAX trip. I am pretty sure (but not 100%) that I checked in online right as OLCI opened. IIRC, 61J was also available at that time.
And I was just able to get 61J for my return, as a Bronze EC reserving a seat about two days out. It was the only seat in row 61 that was available. But when I checked yesterday, it wasn't bookable.
Hope that helps.