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If it's anything like the 2bbl replacement TBIs that Holley sold as "interchangeable, emissions-compliant" substitutes for the original TBIs, there is an "old" injector style and a "new" injector style.
If the injectors have a rounded, rubber cap over the top, they're the old style.
If the injectors have a molded plastic connector for a plug-in harness, they're the newer style. Holley has replacement harnesses. Not cheap!
If you have the newer-style injectors, look at a Holley harness p/n 534-167.
I had to order one of these for my 502-6 replacement TBI unit. I also ordered a pigtail to plug into the 4-wire molded plastic connector which would then splice into the OEM harness.