Like for example, when was the vocals recorded?
Alan Boyd has previously stated on this site that the version on the new box set dates from "circa 1974." The liner notes in the box use this date as well. Alan also noted here that Dennis cut a demo for the song earlier in 1971.
Andrew's Bellagio site has that demo date listed as March 15, 1971. He also has a date listed of Oct. of 1974, which one would assume is directly related to the same "circa 1974," proper version that's on the box.
Thus, 1974 seems to be the prevailing wisdom, with October possibly being a pretty darned good bet as well. As Alan also noted on this site (and also stated in the box credits), it's Dennis, with Carl and Ricky on vocals
(which would've been right around the time that Ricky left the band)