Difficult question!
But I am not sure whether you can speak of P. nagyi “Kuantan” anyway – if you don’t know where they come from. I think in this case you have to say P. nagyi aff. “Kuantan” – as you already suggested. To indicate that they look like the Kuantan-form – but that you don’t know the location.
Calling them P. nagyi “Kuantan” implies that you are sure where they come from – like it would when you called them P. nagyi “Pekan Nenasi”.
I think it’s like my P. rubrimontis aff. “Mimbon 2008” …………. they look like the form from Mimbon imported in 2008 – but it’s not sure if they indeed come from this location.