In quite a lot of music as written, especially in "fake books" and the like as used in "pop" music and to some extent in "big band" stuff from the not too distant past, the convention has been that a "bare 7" means the minor 7th note rather than the major 7th note is added to the triad.

This is something I've found a bit irritating since it's simply "wrong notation," (according to the text books) and some publishers¹ have made an effort to write the scores more accurately - or at least to indicate when what's really written is what is really meant; but the practice of omitting the "m" (m7) for the minor note persists - according to what appears to be prevailing opinion.

For those who believe in this particular mythology, it's usual to assume a major 7th only if it's notated as maj7, and a "bare" 7 means m7; but especially when "academic musicians" try to mix in with working stiffs rather heated arguments can result.

The only reliable way of settling such disagreements is "whichever sounds best" to the one who plays it. If it actually does fit the music the argument that it's "hard to play" doesn't hold much 'possum grease, much less any water at all.

¹ Mel Bay isn't one of these, unless habits have changed since I looked at much of his stuff.

John