The last letters do look the same as the “ly” in affectionately but I think perhaps it might be “tz” my best guess in Hans .... burtz but “kylburtz” doesn’t make sense.
I love the back story though - so “Jack” is her son, not husband. It does sound like the letter writer is replying to ease a mother’s worry for her son. (I’m assuming The backstory to Jack is that he is in the military, and will be based at a camp outside Ismailiah after serving in India)
It’s possible the writer was/is also in the military having travelled on the same ship before but he could have sailed on her prior to its military requisition.
Who’s child is Millicent? You say Ada’s niece but who is ada’s sibling that is also Millicent’s parent?
I’m wondering if the writer is somehow connected to the railways in Egypt, maybe for work? He certainly seems to expect to go to Ismailia at some point in the future. (As does Maisie - and it sounds like they have both been there in the past - I can’t work out if they would be there together or separately)
And if Maisie is going to Ismailiah on her own, what on earth would she be doing there on her own in the 1920s - you say Ada’s daughter was unmarried so maybe it is the same one.
This is wonderfully mysterious and I hope you work out who he is.