Hi Dave,
If you do a search on my posts on Soul Talk, I asked about Jo Ann Garrett and if she did any recording in Detroit for the Duo label because someone (not on here) had mentioned that she recorded there due to the numerous detroit names credited on that label.
I thought she only recorded in Chicago but decided to look more into it ask.
Anyway, more to the point I got a very interesting reply from Robb Klein, from Holland, who lived in the states for a few periods over the years - He was also involved with the Airwave label in the late 70's:
ALL Duo records' recordings were done in Chicago. Duo Records was located in Chicago. Bridges, Knight and Eaton were working more in Chicago than Detroit in the late '60s for Chicago's Ric Williams' labels (Zodiac/Aquarius/Boo/Shock). Mike Terry came to Chicago for a lot of recording sessions (for Williams and Jo Armstead's labels). ZTSC was the code for pressings at Columbia's Midwest pressing plant, with mastering in Chicago, and pressing in Terre Haute, Indiana. Despite pressing about 78% for Detroit labels in the mid to late '60s, it pressed about 20-22% Chicago area labels. Andre Williams was originally from Chicago, and had returned to Chicago in the early mid '60s to do more work there than in Detroit (although he was commuting back and forth between the 2 cities, as were McKinley Jackson, Mike Terry, Mike Hanks, Don Juan Mancha, Bridges, Knight and Eaton and several others). Also, many Detroit session players were brought to Chicago (by Mike Terry, Mike Hanks and others) to record, including many of Motown's (which is why many Chicago cuts sound like Detroit).
Jo Ann was located in Chicago and recorded all her songs there. It is true that Ric Williams had sent Bridge, Knight and Eaton to Detroit to record Ruby Andrews' "Casanova". But all the Duo cuts (Garrett, profiles, Cheryl Swope) were recorded in Chicago (as far as I know).
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So, it's a good posibility he may know a bit more on this subject - I don't think he comes on here though, but I would be tempted to go on there and ask him.
Hope this helps