What is the duality of body and external name-and-form? What is the duality of body and external name-and-form?

What is the difference between the two? What is the relationship between the two?

From the Balapandita Sutta (SN 12.19) (translated by Bhikkhu Sujato):


  “Mendicants, for a fool hindered by ignorance and fettered by craving,
  this body has been produced.
  
  “Avijjānīvaraṇassa, bhikkhave, bālassa taṇhāya sampayuttassa evamayaṃ
  kāyo samudāgato.
  
  So there is the duality of this body and external name and form.
  Contact depends on this duality. When contacted through one or other
  of the six sense fields, the fool experiences pleasure and pain.
  
  Iti ayañceva kāyo bahiddhā ca nāmarūpaṃ, itthetaṃ dvayaṃ, dvayaṃ
  paṭicca phasso saḷevāyatanāni, yehi phuṭṭho bālo sukhadukkhaṃ
  paṭisaṃvedayati etesaṃ vā aññatarena.


The same excerpt translated here by Bhikkhu Bodhi:


  “Bhikkhus, for the fool, hindered by ignorance and fettered by
  craving, this body has thereby originated. So there is this body and
  external name-and-form: thus this dyad. Dependent on the dyad there is
  contact. There are just six sense bases, contacted through which—or
  through a certain one among them—the fool experiences pleasure and
  pain.


Also, as reference, from SN 12.2 (trans. Bodhi):


  “And what, bhikkhus, is name-and-form? Feeling, perception, volition,
  contact, attention: this is called name. The four great elements and
  the form derived from the four great elements: this is called form.
  Thus this name and this form are together called name-and-form.


Also, please note that the term "kāya" has been used in the sense of physical body for example in SN 22.56 (although I know that it can be used to mean group or collection, when combined with other things):


  eye, ear, nose, tongue, body, and mind consciousness.
  cakkhuviññāṇaṃ, sotaviññāṇaṃ,ghānaviññāṇaṃ, jivhāviññāṇaṃ,
  kāyaviññāṇaṃ, manoviññāṇaṃ.
This is just an elaboration of D.O. Everything is clear and makes sense.

Delineation of feeling depends on the notion of contact. 

Delineation of contact depends on separating totality into "internal" and "external".

Six sense doors are the subjective boundary across which the contact is made. 

On this side of the boundary are upadana-skandhas - stuff we appropriate as I/mine.

On the other side of the boundary are the namarupas, the mind-made delineated entities that we designate as "external".

In each pair of DO, it's always a notion and its counterpart serving as foundation.