Actually, I woke up this morning to a bit of a light-bulb moment along the lines mentioned by Euros1951. Indeed, the Ambassador/Senator terminal has five buttons:
On a Senator, these five buttons each select an external line.
On an Ambassador 2+4, the first two select outside lines and the remaining three select intercom signalling to the other three extensions.
So, the thought went... what happens on an Ambassador if I type "83", "84" or "85"? I therefore tried it out with the one I have lashed up at present with a terminal on extension 1 and the external adaptor on extension 2 - and bingo! "85" from the external extension dials extension 1. I'll wire up another couple of terminals to extensions 3 and 4 later and confirm that these are signalled by "83" and "84" respectively.
Edit: actually, I should have said that the terminal has five
selection buttons. It has more than five in total, but the other two are for "divert" and "hold". Who said I'm pedantic?