I think I need a #920 head

I started taking my 273 apart tonight and noticed I have 2 different cylinder head casting numbers. One was #2465315 and #2658920. Everywhere I checked I found the #315 head to be for 64-65 273 although it doesn't look like the bolt holes have been redrilled and tapped for the smaller bolts at a different
angle. The #920 head is listed for a 66-67 273, 67 273-318,or a 66 273-4 bbl
depending on which website you believe. I don't know for sure which head is correct for my 66 although the #920 would be the most logical. I'm going to have to check the #315 head closer to find out why it doesn't have the smaller threads and different bolt angle. If you guys have any 273 heads laying around
let me know if you have any #920's. I may be in the market. toolmanmike