I would start by asking your local dealer.It's possible that they are all the same. Most of the filler cap keys appear to be.

Do you have a spare? because there are plenty of cutters on the internet. eg: https://www.replacementkeys.co.uk/unionfs-series-keys-to-code.html?gclid=CP-VwojxuMoCFU-6GwodcIIMzA


DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4