I thought it was an oil pressure switch but when I tried to order one was told that they're not fitted in the heads only the block.
Its a 3/8 x 1/2 unc with a 3/4 hex head and a single male spade terminal (tip to tip only 1" long)
Its fitted to my 79 305 (don't laugh) in the rear of the right hand head (viewed from inside the car)
Any ideas what it might be???
Thanks in advance, bunj.