The correct setup is the two white rings on the side of the target. Unfortunately, you have the wrong post installed there. PBR does sell them. All of the SM on IPDB have the white rings, as does mine. There should also be a white ring installed underneath the bottom apron. This pic is from my Aztec, but all ‘76 & ‘77 Williams EM’s had this installed. You could move one of the black ring post underneath the apron.

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I checked with three other people with SM, the black post at the top seems to be the norm. Maybe somewhere during production a change was made one way or the other. I’m guessing the black post came first, then they switched to the white. I am assuming this since my Big Deal has the same target setup with white post.