There is a NOS right hand mirror installation kit on eBay---not mine---auction number 225341066242. In the listing is a photo of the template used for drilling the holes. It will take some figuring to see if it will give you the information you're looking for.
That kit looks like a mirror bracket for west coast style mirrors. Backing plates and small angle brackets to bolt the mirror arms to. Checking the parts manual for that PN says it's for a large extendable mirror that would be used on a big bolt.
Jack, It's OK. Sometimes I forget my memory pill too.