On autoatlanta.com site they have a part number that is 2,4-25,3. I'm thinking this is the width and the inside diameter. If anyone has one out and can confirm this for me that would be awesome. I can purchase this o-ring locally for a couple dollars. Auto Atlanta's $1 seal we'll come up to about $16 with shipping.

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Try 914Rubber.com

don't forget 914 318 261 00 sealing ring ..... that's the tiny radial seal
which is often overlooked.... change it.
the O ring is 999 701 064 50 ........ 25,3-2,4
That means 25.3 mm diameter and 2.4 mm cross section

There's three seals for the speedo drive. The small radial seal that Rory mentions, for the drive shaft, an aluminum seal ring that goes in before the drive in the pics, and then the O ring. I just replaced all three on a customer's car. There are modified drives sold (914 Rubber I think) that have an extra O ring

just buy it from your local Porsche dealer.

And that aluminium washer is $10.
I cant find a commercial one of that size.