Using x-sophos-spx-encrypt is there a GPG version?

I have an Outlook addon that I developed to help our users tag emails with some basic classification and since finding that I could combine this with the UTM to get it to SPX emails by using the header "x-sophos-spx-encrypt".

This works well, but I'd like to know if there were any similar header features to encrypt the message with GPG? eg. x-sophos-gpg-encrypt or x-sophos-pgp-encrypt

I know I can put {crypt} into the subject line, but I'd rather avoid that if there's a header based version available.

By way of example:

            Outlook.PropertyAccessor propertyAccessor = msg.PropertyAccessor;

            propertyAccessor.SetProperty("schemas.microsoft.com/.../X-Sophos-SPX-Encrypt", "yes");

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