Just as another data point here, I picked up a new AC-Delco O2 sensor yesterday for the '93 SSEi and found some interesting details:
1) There'* a 251xxxx part number (don't have the receipts in front of me right now) that comes in a cardboard box as a retail replacement part, for about $43 or so. When I asked them to verify the numbers, they pulled out the paper AC-Delco catalog and found additional listings for the dealer replacement sensors, which come in plastic bags and are about $3 cheaper as a result.
2) In the plastic-bag packaging, either AFS-16 or AFS-20 are acceptable for the '93 SSEi. They had AFS-20 in stock so I bought that, and have the empty bag here in the office with me: it carries part number 25162693.
One interesting difference between the boxed AC-Delco sensor and the bagged AFS-20 one that I bought was that the boxed sensor has a large tubular metal collar on it that'* almost the same diameter as the hex collar itself, whereas the AFS-20 has a narrow-diameter collar instead.
Also in comparing the old Bosch unit that I removed with the new AC-Delco unit going in, I see the Bosch unit has much narrower slits on the sensor cover; the AC-Delco unit looks like it should get better gas flow through it. Maybe that partly explains why people complain about slow response from Bosch units.
-- Andy Green