The Friends rolling stock roster entries for locomotives 484 and 483 indicate that the first train was a New Mexico Railroad Club charter on June 26, 1971, and the first public excursion was on Saturday, July 3. The sources of this information are the Tell Tale Vol. 3 No. 4 (the Tell Tale was the original volunteer group's newsletter), the Chama Tattler Vol. 11, and the C&TS Dispatch Vol. 9 No. 1. I'm traveling so I can't look up those sources directly right now, but I could see what else they say when I return.