But the M58 didn't come about until 1964 ~ 5 years after barrels were no longer stamped. Also it does not look like factory stamping to me, being along the bottom edge and not semi centered horizontally or vertically, let alone it's kind'a sloppy for factory stamping. But nothing the factory did and does surprises me any longer.
The long answer with context on serial numbered barrels is:
Decreasing Serial # locations: The number of serial #s or if model # is stamped on a particular S&W Hand Ejector has more to do with where it was in the production/assembly stages when change orders were issued, therefore as we've learned to expect with S&W, there are great variances and exceptions galore.
Officially, on May 1, 1957 S&W eliminated the Soft Fitting Operation: So it generally corresponds with model numbers ordered June 12, 1957. It was no longer necessary to routinely stamp the serial number on the barrel, cylinder & yoke arm rear surface and show up unstamped over a transition period. So guns in process or in inventory as of 5/1/57 can still have more than 3 and up to 6 locations, and guns shipped after this time may have some of the former number locations because assembly was done over time and as inventory from the old process was used up.
The 6 serial # locations were down to only 3 left on the majority of models (but not all) from c. late 1957 to 1959 which are:
Extractor star - backside
Right stock — backside
Original Serial # locations: Here are the 6 (or 7 on Triple Locks) pre war fixed sight frame serial # locations and locations remaining after WW II thru ~1956 to look for:
1. Gun butt* - or forestrap* on I frames/single shots with grips that cover the butt
2. Barrel - bottom of barrel or in extractor shroud
3. Yoke - on rear face only visible thru a chamber with a flashlight
4. Extractor star - backside
5. Cylinder - rear face
6. Right stock only** - on back (except most post war target grips because individual fitting not required.)
stamped, scratched or penciled depending on vintage and stock material.
7. Mid-lock cam plate – “Triple Locks” only, in any caliber (up to all 5 digits).
*NOTE: The one TRUE place you can be sure of reading the original serial number for all Hand Ejectors of any vintage with stamped numbers, (which includes any letter prefixed #s after WW II,) is the BUTT of the gun, (or front grip strap on non-round butt .22/32 Kit guns and Targets, .32 & .38 S&W Regulation Police pre Model of 1953 I frames. And the 32 Transitional Targets from 1957).
The number on the butt may be drilled thru by the factory for installation of a lanyard swivel but is re-stamped on the grip frame, under the left stock. Factory installed swivels are always 1/10” forward of center.
Generally the Pre war serial # on the butt reads with barrel to the right including I frame serial numbers on the forestrap. After WW II the serial # reads with barrel to the left (except for serial numbers on the forestrap thru 1957).
Pre-War serial #s are centered on the butt, unless there was a swivel. Post War serial numbers are offset.
** Stamped since 1857, stock #s, almost exclusively on right panel only, changed to penciled #s c. 1900 and back to stamped #s in 1929. Scratched, penciled or stamped on hard rubber and premium stocks; numbering discontinued ~ late 1970s. Pre war penciled S/Ns are in the top half of the stock near the backstrap and read with the stock oriented with the back edge down. Post war numbering switched to lower right half of grip; earliest observed ~1960.
Pre war target sighted guns have serial #s in the above 6* locations, target** models as many as 9. Triple Locks have 7 including the mid lock cam plate, therefore 10 locations on Target models.
*Post War continued the 6 fixed sight locations thru ~1956, and 9 s/n locations on early Transitional target models with pre war target sights only, thru 1957, but dropped the 3 target sight locations post war on the new Micro-click sighted models, which were no longer specifically fitted to the top strap.
**Target models will have the serial number on:
the front sight,
under rear sight, and
the rear sight blade, although the rear blade can be #'d with the assembly (factory work) # instead of serial #. If the s/n is more than 4 digits, the front and rear sight blades can have a partial s/n. All three parts must be removed to observe the #s. Non-numbered front and rear sight blades have typically been replaced, in most observations.
We owe the Russians a vote of thanks; having been the 1st to require multi-serial # locations on their S&W #3 contract revolvers.