Is it a genuine Honda locking cap?
They are very hard to find in the UK but fitted standard to many German bikes. Due to the fuel breather being out of the side of the cap they can be used with a tank bag, so useful when travelling even if you don’t use the lock.
I have an after market locking cap which does the job but the quality is poor and I’ve had to fit 2 rubber seals to stop it leaking.
It’s good news you didn’t need to damage it.