I'm wondering if Manual Transmission vs. Automatic Transmission might explain the 4K vs. 100K knock sensors. The 1227747 ECU has a knock filter module mounted near the throttle body. 1991 also used the 16147060 TBI ECU / PCM with 4L80E transmissions. The 1991 16147060 PCM uses a Memcal inside the PCM that has the knock filter instead of a knock filter mounted near the throttle body. Maybe the knock filter specs you posted was for the knock sensor used with the 16147060 PCM?

I'm thinking the bore size is how the knock sensor frequency is determined.

I would first try the 100K knock sensor on the Pontiac 350, provided the external knock filter is being used. I'm thinking the 4K knock sensor would NOT be the correct knock sensor for the 1227747 ECU.

dave w