dfaris brings up an interesting tidbit of info regarding the differences in cylinders on the MKIII.
Another interesting difference between your early production 6" MKIII & late production is the front sight & the top strap. Late production has a gradual ramp upward toward the barrel with the front site perched on the ramp. The older production had a flat top strap with the front site mounted on a pedestal base. Yours should be the latter.
So does any one know what model years these changes took place?
I figure the answer will come from someone with a large MKIII collection or a great reference book. I can narrow it down to a range of years for each trait based on my MKIII collection but not sure of the exact years the changes took place.
I did double check the cylinder and it is counter-bored a la S&W pre 1981. The front site is mounted on a pedestal base and the entire top strap of the barrel is flat.
However, I compared this Trooper Mk III in 357 with the J73xx SN to my Trooper Mk III in 22lr with a SN of Y87xx circa 1980 and the front site arrangement and the entire top of the barrel with the flat strap the whole length are identical.