Something doesn't jive here. First there were four types of barrel shrouds for DW revolvers. 1) Regular - ejector rod shroud but no underlug, solid rib on top. 2) Heavy - Indicated by "H" - a full length underlug and solid rib on top. 3) Vent - Indicated by "V" - upper ventilated rib, no full underlug, just an ejector rod shroud. 4) Vent/Heavy - Indicated by "VH" and had both the full underlug and the ventilated rib.
The knurled barrel nut went away with the advent of the dash two (-2) series and stainless guns (7 series), AFAIK, post dated that change and therefore were never made with the knurled barrel nut. Also DW barrels were 6" not 6.5" and while I believe that some of the later, heavy silhouette calibers may have had some type of factory porting, I don't think the standard .357's did.
Several possibilities - Aftermarket ported barrel & shroud using the old-style knurled barrel nut. Or old barrel with new shroud, using the old style barrel nut (not even sure this is possible - the knurled nut barrel shrouds looked like a pork chop - they were "L" shaped with a vertical leg with a locating stud that went down the front of the frame).
EWK Arms would be the most likely source for aftermarket barrels and shrouds...
EWK Arms
Photographs would help.
Disclaimer: All of the above is from my aging memory so may also not be 100%. I believe there is a DW forum which is probably the best place for questions about this gun.
Good luck.
Adios,
Pizza Bob