I have never gone to any depth into this, nor have I now – I am contributing my bit only because Lal requests it.
My first impression is that the physical body is external rupa in the sense that it not under our control, or so only to a minimal extent.
That is, with pasada rupa, once arisen, say sight ,it is then up to us to welcome or to abandon it. But the functions inside the physical body, say those of the digestive system or breathing, to give two instances, work perfectly well without our being even conscious of them. In that sense the body may be said to be external rupa. Also, at death, we take with us ‘all that made us up’ in the non-material sense, but the physical body we leave behind as ‘external’ to our true being.