Not sure if I got the correct spot.
A plate fits barely inside this, and it is slightly angled towards the inside, so it might be correct?
at the top it seems to be slightly more than 3.5mm (8.75 LDU). I don't know how to measure this more precisely without risking damaging the part.
I am also not sure if this matters that much. All that is important, is that it should fit the matching protrusion on the Toa Mata head there:
A plate fits barely inside this, and it is slightly angled towards the inside, so it might be correct?
at the top it seems to be slightly more than 3.5mm (8.75 LDU). I don't know how to measure this more precisely without risking damaging the part.
I am also not sure if this matters that much. All that is important, is that it should fit the matching protrusion on the Toa Mata head there: