There is no actual evidence. Toledo, Ohio, is essentially part of the Detroit Metropolitan Area. Almost all singers and groups from there recorded in Detroit. For every one from Toledo that recorded in Ohio, I'm sure that we could list 10 that recorded in Detroit. The liklihood is that they recorded in Detroit. However, you are correct that we have no evidence, whatsoever, that they recorded at Magic City. Mel Collins worked for Giant of Chicago. However, they recorded Ruby Andrews in Detroit, and had Mike Terry arrange other recordings of theirs in Detroit. If I had to bet money on it, I would guess that The Four Tracks' songs were recorded in Detroit. I would NOT bet money that it was recorded in Magic City Studio. Upon listening to it several times, again, it sounds not-at-all like the acoustics of soungs I know to have been recorded at Correc-Tone/Magic City, and the session players certainly don't sound like The Funk Brothers or any of the best Motown/Detroit musicians of the time. The 2 cuts sound like they were recorded in one of the low budget Detroit studios of the time. We know it was pressed in Detroit. It might also be possible that the vocals were recorded in Toledo, and the instrumental in Detroit (or Chicago, for that matter).
Yes, "Glass City" refers to Toledo, which was known as "The Glass City", as that was a primary industry there. So, we have already admitted that Mandingo Records was a Toledo, Ohio company. But, as stated above, most Toledo based labels did their recording and pressing in Detroit, and most of their artists played most of their gigs in Michigan, rather than in Ohio. Toledo is to Detroit as East St. Louis, Illinois is to St. Louis, Missouri, and East Chicago, Indiana is to Chicago, Illinois, and Kansas City, Kansas is To Kansas City Missouri, and Newark, New Jersey is To New York, and Paducah, Kentucky is to Cincinatti, Ohio, and not even unlike as Windsor, Ontario, Canada is to Detroit, Michigan, USA.