Well, Lord Sandwich, I find this quite odd. You say it is "early in the S prefix range" but then you say "no apparent prewar parts." I have more than 5 dozen accounted for in my database now and all of the "early S prefix" 4" and 5" examples have large knobs and patent dates, at least. I'm wondering what you mean by "early." Would you care to share the serial number with me? If you purchase this gun, would you please contact me so I can add it to my research? I'll have a list of questions and would also appreciate photos.
As to the value, I think the guys are right about the $500 ballpark, but the box might add to that a bit, especially if it is in good shape and numbers to the gun.
Regards,
Jack