Tach Gage Cluster- What year do I have?

[ChaseDale3and8]

Does anyone know for sure what year this set is? I'm thinking that it has to be from a 78-79 since the 77 that I used to own had a mechanical oil pressure gage and this set has an electronic oil pressure gage and I believe that the 1980 and later kind of looked like the gages that I have in my 86.

I have been told that these will work in my 86, but of course will not be correct. I need to order a tach wiring harness from Brothers Trucks, but I need to find out what year these are so that I can order the right one. ...Joe