Actually, the three top mounting holes are normal for '80-'91 trucks with the 7x10 trailer mirrors; they switched to using two holes (the same as the "baby" tripod mirrors") in '92. I know that the "baby" tripod mirrors on '84+ trucks have only two mounting holes on the top, but the larger mirrors share those two holes, plus a third. Your setup may be similar, or it may not be.
I took these pictures to get an idea of the mounting point location on both of the "later" mirror setups...maybe they'll help you some? I only have one pic of the 7x10 mirror setup, but I can take more if you'd like...
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