Why 10.7 volt input at ‘71 Duster instrument panel?

I just posted this question in the transmission forum - but thought I’d ask here since I’ve already engaged you guys on my speedo issue. I purchased a 34 tooth speedo gear which will replace an

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IMG_0183.jpegincorrect 40 tooth speedo gear. When I pulled the speedo gear off I found a good orange seal plus what looks like disintegrating black sealant around the outer shoulder. I installed the orange gear in about 1987 and do not recall the story on the black stuff. Does the speedo gear holder require an outer seal or is the orange seal all that is needed?

FYI - I ran the speedo with a drill from below the car and the speedo is not clicking. There was no apparent damage to the orange gear.