[PD] external with more than 6 inlets possible?

[i go bananas]

I'm scratching my head wondering what's happening with an external i have
written.  The first 6 inlets work as expected, but not getting any response
from inlets 7 and 8.
I cut my code down as much as possible, and made a really simple external
which just adds signals and outputs the result, but still getting this same
behaviour.

In the howTo guide on writing externals, it mentions that an object can
only be given a maximum of 6 arguments, or else needing a GIMME, so i was
wondering if perhaps that 6 argument limit is somehow related here?
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