Funny you should post that question, I was just thinking about the DEFENDER 38 Models and
their relatives. Here's the scoop.
Circa 1935 H&R decided to expand the utility and appeal of the SPORTSMAN series of large, hinged frame revolvers. One of the first "moves" was to cut the barrel down from 6" to 2.5" (NEW DEFENDER) and 3" - keeping the caliber at .22 rimfire. Neither of these seems to have "taken off" - the NEW DEFENDER is, today, somewhat scarce and the 3" barreled SPORTSMAN are down right RARE.
Another tack, involved increasing the caliber to the "threshold" of power for the hinged frame guns which was, then, the .38 S&W smokeless powder cartridge. The first examples were equipped with 2" barrels (extremely rare - if actually existent) and 4" barrels (very scarce) and called the "DEFENDER SPECIAL" Model 299. Equipped with the same adjustable front and rear sights as the SPORTSMAN parent.
Some refinements were made to the "DEFENDER SPECIAL" between its inception circa 1935 and 1939, resulting in a couple different looks or variations, on the base platform.
Then in or around 1938 - 39 period, recognizing that a European war was imminent, H&R began developing another version of the DEFENDER, based upon the SPORTSMAN lower frame, but with a modified upper frame which has features of the AUTO EJECTING series - i.e. it had a fixed, semi-cicular front sight and for the rear sight, two small projections arising from the barrel latch. This crude sighting system is identical to that on the AUTO EJECTING series of this era - and seems to be a throw back - when compared with the DEFENDER SPECIAL sights - this sight system was probably a cost containment measure. This "DEFENDER" series doesn't seem to have been very popular although widely advertised at the time. There is no record of a model number having been assigned to this variant - 4" and 6" barreled pieces are known, both of these are NOT marked on the barrel with a model name or number, but match pictures found in a period advertisement. This variant appears to be very rare, although serial numbers indicate several thousand may have been produced.
The need for a "secondary" police/security side arm became apparent as the Eurpean war picked up intensity and many in the US arms industry realized the opportunity to provide firearms to fit the needs of the belligerents and also for use by the USofA, who just about everybody realized would eventually be "sucked" into participation. So, from that, H&R made some further changes and refinements to the "DEFENDER", adding the fully adjustable front and rear sighting system as well as a pinned, replaceable firing pin on the nose of the hammer, added a "gold" sighting dot to the front Patridge type sight blade and the "DEFENDER 38" series was born. This series was labeled as the Model 25.
The Model 25 was made through out the war years and probably as late as 1947, when it morphed into a manual ejecting model, which H&R labeled as the model 925. The re-issue of the Model 925/926 manual ejecting models in the 1960s is another story.
Defender 38 will have a four inch barrel (5" and 6" were catalogued but I've not seen an example of either), have blued finish (early examples are highly polished, later examples will show a deep bluing but the finish is slightly striated and not as highly polished), trigger guard will not have the finger rest, front sight is height adjustable with gold bead insert, rear sight is windage adjustable, all WERE DA/SA and came with molded, checkered black (some are seen with dark brown) Nylon derivative, one piece mono-grip on the RICE type frame.
Your DEFENDER 38 with s/n 6407 is, most likely of 1943 manufacture - there is only one, currently known, definitive date associated with a serial number for the model. I have a NIB, unissued Model 25 DEFENDER 38 that came with an inspection tag, serialed to the gun (13533) and dated May 8, 1944. Production of these handguns during the war does not seem to have been in very high numbers probably less than 15,000. H&R was concentrating on production of other goods and weapons during the war as directed by the Defense Procurement agencies, and handgun production of all types was severly curtailed. It is not known if any of the DEFENDER 38s were supplied to allied nations or employed for military use - although, H&R did provide numbers of other models to the US Gov't for special uses as well as a variant of the Auto Ejecting series to England for use by the London/Metropolitan police (over 23,000 pieces) - these are called the "Bobbie" model and some are seen that were intended for domestic use, as well.
Your gun is a bona fide example of a WW2 "secondary" revolver, mainly intended for domestic, civilian security and para-police use.
Edit: The letters stamped into the grip frame, under the grip, seem to be internal H&R inspectors' marks - they show up on many of the guns from that era and a variety of letters are used. I have no idea what the numerals '16' indicate, other than it is possibly a rack or inventory number applied by an agency or company that secured a number of these (as well as other weapons) for security personnel.
Here's a bunch of pics to illustrate the above narative (NOTE: the dates on some pictures do not reflect current "thought", also, the STAG grips on the 3rd Var. piece are not original to that gun - they were used in the late '40s and early '50s on other models.)