The odd thing though, Roger, is that the RCA matrix numbers starting with T signify 1966! The stereo version in Discogs has matrix numbers starting with U, which is right for 1967. I wonder whether Motown's documentation is deficient, and if this mono version actually was a 1966 release, and that there were some further vocal and instrumental parts recorded in 1967 for the stereo version (or just alternate versions for stereo)?
I don't know where the supposed release date of May 1966 comes from, all the Motown discographies that I've ever seen have it coming out at the same time as the Stereo version in August 1967.
Furthermore, the booklet notes by Keith Hughes in the expanded CD version of this (released in The UK by Kent as "Truly Yours" in 2012) have the band tracks for "For All We Know" and "I Cross My Heart" not being recorded until late 1966, with The Spinners not adding their vocals until early 1967. A lot of these details are on the Don't Forget The Motor City website.
I think the confusion arises as it looks like Motown 639 should have been released around then, as that would fit in with other releases. I suspect that this catalogue number was set aside for a release (possibly a Spinners LP) but cancelled and reused later.