Yes, contracts are out there but those who have them are not getting the research importance. Blue Note has contracts which would prove when an artist was signed to them but they are hanging on tightly to this info. It is needed whenever a tape shows up and it has to be determined which label, if any, has jurisdiction. This will be an issue if that Grant Green Left Bank tape ever surfaces. I think he was not with Blue Note, but they may claim otherwise.
The Left Bank archives at UMBC does have a few contracts, let me see if I scanned any of that when I visited last August. I know the fee for a Duke Pearson Big Band gig was shown, and maybe a couple of others.
To watch someone in action claiming contracts are of no historical importance, please watch the YouTube panel discussion I was on last year. You should be able to find it by searching for 'Baltimore Jazz Alliance Left Bank Jazz Society'.