This is a a Personnel Parachute Harness for PCU-17P. On the back is stenciled in Blue: Part No.
68J420-101 HARNESS, PERSONNEL PARACHUTE, CHEST PCU-17/P CONTRACT NO. F41608-80-C-0323 PIONEER RECOVERY SYSTEMS, INC. DATE OF MFG JULY 1980. I hope this info lets you know what you need to know about this harness because I know nothing about it honestly.
I will try to answer any questions you have though!