And 400078 shipped in March 1922, so your gun might be dated to late in the year or early in the next. But S&W had no policy of shipping in serial number order, so if the gun sat in inventory for several months before being shipped, the date of record could be even later.
If the gun doesn't come with its original box, for seven franklins it had better be a 98% or better gun with immaculate bore and not even a hint of rust discoloration under the stocks. The stocks should also be unchipped and still show sharp checking. Can't tell without pics, but I get a vibe that the one you saw might be suffering from aspirational pricing -- or the dealer is inviting a counteroffer. Make sure it hasn't been refinished, but from the high condition of the stocks it sounds as though it probably wasn't. I agree pics would help, but what I see in my mind's eye based on your quick description might be fairly priced in the $475-500 range. assuming the finish is original.