Lost fuel deck filler cap

I've lost my fuel deck filler cap over the side... a Maxi 84, so I'm presuming a Swedish original part (?) Any ideas what the thread would be? (It is beyond me to work this out by looking at the remaining deck fitting) Would a Roca part work?

Fuel Deck Filler Cap with a Flip Style Cap

The original is aluminium so my magnet does not help me....
Thank you for all the sage advice good people, as ever this forum is a great resource. The helpful chap at Apollo Marine in Portland found something to match my water filler (that is the same size as the fuel filler) and is sorting me a replacement. As the wife said - 'Why are you using on-line suppliers? When you desperately need a chandlery it will have closed because you don't use it!' I have had very good service from many on-line suppliers but she is right.
 
When you desperately need a chandlery it will have closed because you don't use it!'
my local chandlery / engineer have sorted so many issues and kept me sailing so much this year, even out of hours and on sundays. I won't spend money anywhere else now, because they have gone the extra mile every time I needed it. Worth the extra it costs me for goods, for the solid gold service.
 
You cannot beat the personal touch. As a relative newcomer to sailing, I have seen how so much depends on opinion, and as someone often lacking knowledge, I value these opinions - which after all I can take or leave. The reassurance of knowledge and opinion, on this forum, at the marina, and often from the chandlery, is invaluable. And the additional cost is not as great as I thought, and well worth paying to keep chandlery services going local to my marina.
 
You cannot beat the personal touch. As a relative newcomer to sailing, I have seen how so much depends on opinion, and as someone often lacking knowledge, I value these opinions - which after all I can take or leave. The reassurance of knowledge and opinion, on this forum, at the marina, and often from the chandlery, is invaluable. And the additional cost is not as great as I thought, and well worth paying to keep chandlery services going local to my marina.
Looks like I am going to eat my words. Waited and waited from the local chandlery to get the part I needed, and now I have given up & gone to ASAP.
 
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