It's too bad the machine that would have been in the treadle with the coffin top is gone. And like Miriam says, that is the only one of the three that's not so common. With the exception that is looks like someone made the opening larger and put a shelf for a much newer machine to sit on. So one probably could not put the original machine back in.
The 66 is probably not a conversion - it looks like it's a newer (40's) that came fitted for electric and in the cabinet it's now in.