Has the gun had the conversion done to the transfer bar safety? If not, even without an extra cylinder or the box, that is still a very good price, given the apparent condition from your pics. The condition of the gun is the important thing, much more so than whether it includes the extra cylinder. In our area, that gun would easily sell for over $400. I'd have no trouble buying it, but it's your call.
That is within the time frame when Ruger started supplying these with extra cylinders, so it MAY have shipped with one. Yes, even on these Standards, some were supplied with extra cylinders, but those often got lost over the years.