my 77 MH 440 uses a 5/8 hex,tapperd seat,no washer,peanut plug,
autolight 23

I will be looking for those casting marks on mine to see what they have,I did see some kind roll pin/plug deal on the end of the head but never paid it any attention

looks to me a autolite 23 will screw right in

gotta go look at mine now cant wait till morning