Confused about 2001 G20 Emission Control

[mediocre mechanic]

I just joined to find help with troubleshooting OBDII code P0420. The thing is, I found one O2 sensor (S1) in the exhaust manifold and the downstream O2 sensor (S2) in the front pipe under the engine. The catalytic converter I just replaced is actually downstream of S2, so of course that didn't fix the problem. But I can find nothing about a primary catalytic converter inside the exhaust manifold or somewhere before S2 and after S1. Or is this SR20DE engine so clean there is no other catalytic converter? The car is automatic with 183,000 miles...Thanks!

[Rogue One]

Infiniti G20 Factory Service Manual