last C-19 in service, last Class 56 engines

Well, 340 was certainly the last C-19 on the roster, going to Knott's in March of 1952, but in service? If by in service you mean under steam and performing work, maybe not. I have seen in several sources that the crews at Montrose did not like 340 very much and that, after 1949, used 318 for almost all trips to Ouray. So I'm really not sure when 340 was last used.
345 was retired in the spring/summer of '51 in preperation for her movie role in July. Again, not sure the last time she was used but I have seen pictures, alleged to be from 1950 and early in '51, of her in use as Durango switcher.
346 Dead in Durango Yard: Just to confuse this, the junker who bought her after the MLCo sale had her hauled to Durango(1948), reportedly thinking he could sell her to the D&RGW. D&RGW eventually said heck no and get that piece of junk out of our yard so she went back to Delores, where she sat until she was hauled back to Durango again for Bob Richardson in '51.
And on the subject of Class 56 converted to 0-8-0, "Steam in the Rockies" (CRRM, 1963) lists #'s 31, 50, 58, 64, & 65. All were scrapped 1916-17, presumably due to Federal boiler law. The few other Class 56's not scrapped in 1908, #'s 30, 34, & 83, were also scrapped in 1916-17. Just for completeness, SitR has no dates (just the note "dism") for #'s 2nd 41(reno from 24 in 1894, the 1st Class 56 engine) & 84.
C-18's 315 & 319 were retired in the Fall of '49, btw.
hank