I don't know if it's the stock fuel cap, but I'd guess yes.
That style of fuel cap is typically hard to replace as they don't come up for sale very often.
Not sure, but I don't think the lock cylinder is "keyed" to a specific code, so any correct style key should open it.
Does the ignition switch key open the seat lock & helmet lock?
I would probably try to get a new/correct key cut to the code on the seat & helmet locks,
since those two are probably the originals. It may not matter but might eliminate some doubt. ???
Also, I could be wrong about any key opening it. but I'd be surprised if that was bad advice. hee.
My fuel cap is the 80s rectangular stye and I have a couple of spare fuel caps & all the keys open mine.
For reference. here's one that I think is the same as yours. I've never seen a key cylinder like that.
Just an FYI. AFAIK, there isn't an aftermarket or universal fit fuel cap available in the correct size.
If you'd like the measurements of the fuel cap, I saved a pic w that info. I'm pretty sure it's the same as yours.
Nerdy originally posted the pic & measurements (but I combined the two).
gotta give credit where credit is due. hee.
Thanks everybody, did get there in the end. Penetrating and thin oil down the key slot over a period of a couple of weeks. Wiggling the key and slightly twisting back and forth whilst relieving the pressure on the cap spring.
The 32 year old gas was varnish, but the neck cleaned up with a stainless toothbrush and some phosphoric. Cleaned out the tank and flushed with further phosphoric and then treated with a clear weld through primer I use here for preserving fabricated parts - Epiphos. Leaves a thin impervious coating and really bites into steel treated first with phosphoric. Tank was not leaking, so I'll not go any further with it - particularly that I plan to make an alloy tank later in the year once the bike is back on the road again. We have a regulation change coming in in October whereby any bikes not currently registered for the road, have to have ABS to protect the numpties who can't ride.