Currently, the certificate details of an S/MIME certificate look like this:

We want to add the serial number here, as it is (together with the issuer) the unique identifier for an S/MIME certificate. I'd say we add it below the issuer.

Should we add both the hexadecimal and the decimal number or only one of them?

And I wonder if we should not move the position of the fingerprint to directly above "Private key", as it is less important info for an S/MIME certificate (we only use it internally and it may change).