The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.

IIRC, circa 1981, this was tested for FCC Part 15 RFI compliance rules when used with a shielded serial cable (which many hobbyists very likely didn’t use).

So expansion slots were not fundamentally incompatible. But it did take a few years before Apple made that available in revisions of the Apple II that included coated plastic, conductive seals, and back plates for the expansion board connectors.

The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.

IIRC, circa 1981, the Apple II+ with Disk drives and a Super Serial Card plugged in was tested for compliance with FCC Part 15 RFI/EMI regulations, when used with a shielded serial cable (which many hobbyists likely didn’t use).

So expansion slots were not fundamentally incompatible. But it did take a few years before Apple made that available in revisions of the Apple II that included coated plastic, conductive seals, and back plates for the expansion board connectors.

The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.

IIRC, this was tested for circa 1981 FCC Part 15 RFI compliance rules when used with a shielded serial cable (which many hobbyists very likely didn’t use).

The Apple Super Serial card did come with a metal back plate for the DB25 connector, which was clamped to the interior conductive coating on the plastic cases of later revisions of the Apple II+ and the Apple IIe.

IIRC, circa 1981, this was tested for FCC Part 15 RFI compliance rules when used with a shielded serial cable (which many hobbyists very likely didn’t use).

So expansion slots were not fundamentally incompatible. But it did take a few years before Apple made that available in revisions of the Apple II that included coated plastic, conductive seals, and back plates for the expansion board connectors.