P0421 and multi-cat-conv's?

(btw i searched and my q is a little different. maybe)

99 Miata 6sp. 1.8L NY car (not CA). 128K.
Cat was ignored for 2 years as a daily driver for years. Cat is done.


Recently installed are new plugs/wires/coilpack/front o2 sensor.
Car seems to pull great right up to the red.

P0421 lights up, twice (cleared inbetween 2 70- mile runs. again, feels great for both long rides.)

New midpipe w/ cat is set to go on ASAP but considering it's after both sensors, suprised to see this light up. so here's the Question

What set this code off if both o2 sensors are before the cat?

Do I have 2 cats w/ a non-California car?

I see the heat shield covering what might be a "3 way warm up cat" (but i thought that was for CA cars only?)

I dont know if the O2 sensor in the back senses a blockage/ accumulation from the 'real' cat behind it, or from the 'mystery cat' in front of it.

(If there is a cat in the rusty corroded header, that's a grand place to put one. ...)

Thanks in advance.

Your'e cat is going to be in between your front o2 sensor and your rear o2 sensor.