It's a "38 Chiefs Special Model of 1953 NEW J frame, therefore does not have a trigger guard screw. It's #72330 is the highest known with the old style hammer.
Engineering change order:
• Eliminate upper side plate screw change order on 12/13/55, the 4th screw on New I (earliest ~ #640980 on .32s) & New J (earliest ~ #73933) frames.
Yours is probably a 4 screw side plate because it was shipped right on the cusp of the change and even produced slightly earlier. If it has the top side plate screw, it's a 4 screw (one screw hidden under the right stock.)
Much more detail here:
FULL SIZE “New J frame” Chiefs Spl including the Airweight:
1953 - Introduced the Model of 1953 Chiefs Special (pre model 36) “New J frame” #47475 is earliest known; larger grip size and larger, egg shaped trigger guard, eliminated trigger guard screw, the 5th screw, (transition period for dropping upper sideplate screw, the 4th screw, started by at least #73933), as well as bright blue finishes. Lowest known # 462XX (with old hammer). The 1st issue of 100 pre Mod 36/50 target chiefs in 1955 serial range 55050-57919 are in this group and have been observed as still having pre war style hammers. Highest known fixed sight #65986 Oct 1955, with early hammer. Highest #72330 with old hammer.
Airweight Model (pre model 37), It’s reported that the 1st Mod of 1953 Airweight was introduced 9-12-1952. Reports indicate that airweights are between 240XX shipped April 7, 1953 (w/alloy cyl), up to at least #131612 (w/steel cyl).
Earliest Airweights in the #24-27,000 range with aluminum cylinders, about 3777 made (lowest known 240XX shipped April 7, 1953 ) (alloy cyl # 43926, shipped in January, 1954, highest known, # 82317, 3 screw). US Air Force batch #AF1795- AF2399. C.1954 changed to steel cylinders due to weakness; a steel cyl reported as low as #43750 & #438XX with old hammer, shipped 1956!
Models of 1953 #573XX & 616XX highest known with an old hammer/fine checkered.