Scubapro used a #13 O-ring for years and it works just fine if you are careful to seat it correctly when assembling the first stage. At some point they changed to a formed seal. I have been trying to replicate that seal for a while now. The latest version below is from my sealing company and was cut by water jet. It works great but I have not added it to the service kit because when water jet cutting the ID does not come out perfect and the Subapro fanatics would cry foul and failure to the high heavens despite how well it works.
I have sent samples of the Scubapro formed seal to a sealing company in Germany that I work with just in case it was some off the shelf metric seal I was not aware of but no luck there either.