You're saying it was cut from a vintage mono mix, implying said mix was final, or near final. Some of the acetates were a very rough mono dub from the multitracks for Brian to consider his next move. According to your statement, Brian had finished mixes for most of Smile by late 1966, and we know that's not so.
Er, my post did not state or imply anything of the sort. I don't see how "vintage mono mix" suggests anything other than a mono mix created around the time of the sessions. Could be anything from a tossed off rough mix of a basic track to a final mix. I re-read my post and I don't see how anything I wrote suggests "finished mixes". Of course any vintage mix, no matter how rough, is of value as it reflects some level of input from Brian.