Well, gentlemen, it appears that many of us with very early (painted) mirrors, in the round and rectangular variety, have slight differences.
This further points to evidence that Toyota may have utilized a number of different manufacturers to provide mirrors for the US-spec 40s. Since they are painted body color, I can only assume that they ordered from stateside manufacturers and introduced them into their assembly process back in Japan, which could add a bit of shuffling from mirror to mirror on the line, and could be evidenced by non-metric thread pitches. (Your thoughts on this theory are welcomed!)
Even early Toyota illustrations show a variance in the style of the back of the mirrors (smooth back, pressed cross back). It's hard to tell from the early illustrations and photos, but it also seems that some have rivets and others have screws.
I'm leaning more and more toward this: Toyota acquired (perhaps from US manufacturers) a variety of mirrors and installed them on our early (1961-1965) US-spec 40-series Land Cruisers, but did not yet manufacture them themselves until later years.