The square ring is supposed to sit flat on the cover. As you tighten it the gasket is forced into the chamfer to make a good seal. When you pull an old one out they are triangle shaped - because they are forced into that shape. If you have issues with leakage then use a thin coat of lock-tite "Gasket Maker" onthe square ring before installation.
There are different square seals that are in the kit but only one fits the rear servo cover.
The rear servo only gets pressure in reverse and manual low. (unless you have an aftermarket valve body)
With manual aftermarket valve bodies it is usually the same but it can also only be pressurized in reverse.

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Paul
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retired mechanic after 35 years
specialized in Holley carbs and Ford Automatic Transmissions