Let’s start off with the fact that you do not have a stock valve cover, but an atermarket finned aluminum one.
There are (2) different caps ffrom Victoria British - the first is VB part no. 17-697, which only fits the cover without a crankcase breather tube ($9.95), which appears to be what you have. The other is VB part no. 17-696 which is for the other covers with a breather tube ($29.95).
Now, is either for the cover that you have? You might also try eBay, but the same issue arises as these covers are aftermarket and, though the sources are limited, they could be very different.
It might just be easier to get a new valve cover with cap. Used on ebay, it might be cheaper than the more expensive cap from VB.
Not sure why the big difference in price, but you can find both along with the different covers on page 56 of the current VB catalog or on-line at
www.victoriabritish.com. I hope that helps. Let me know how you make out.
Safety Fast!
Art Isaacs