Mirror mount holes

[Hawssie]

I know this is a minor thing but its kinda bothering me. I talking about the 3 holes in the door for mounting side mirrors. I have 4 trucks/blazers for reference. I have read 1970 was when they started using the small square sport mirrors without the long stem. And they seem to mount right about in the middle of the division bar between the vent and door window. But out of my 4 vehicles my 69 has the 3 triangle bolt holes for the long stem mirrors which are located farther forward toward the fender, as it should but so does my 1970 C10 and one of my 1972 blazers. My other 72 blazer has the small stem mirror in the right spot. I am wondering about the other 1972 blazer and mostly about my 1970 C10 as thats the one I need mirrors for. I guess I will have to weld in the 3 holes if I want to use what I thought were the right mirrors for it. I was just wondering why it didn't have the holes for these mirrors there in the first place instead of the ones for the 1969 down or am I missing something. I know I can mount any mirror I want to but to be correct was the long stem mirror still installed on 1970 models? I should mention the 1970 had camper style mirrors on it when I got it. The other Blazer I cant remember what it had(it has nothing mounted now, was stripped long ago for paint but it is a CST)