Image won't display with given name Options

[Robolovsky]

I have been coding in HTML for many years and do not understand what is happening.

Please take a look at the following test web page: http://www.snookerlegends.co.uk/imgtest2.html

The page contains two identical images (with slightly different names), each inside a table border. The top one does not display while the lower one does.

The top image is called FP_ad1.jpg and the lower one is called FPad1.jpg (no underscore)

If you check the page source and click the link to FP_ad1.jpg you will see that it does exist on the server so why does it not appear in the page?

The Doc type is HTML 5 but I have tried it with many different Doc types and always the same result.

Even stranger is that if I add any other character after the 1 the picture then does display so FP_ad10.jpg will display

However if I remove the FP and underscore completely leaving ad1.jpg that does not display while ad10.jpg does.

I am completely baffled by this behaviour, I have tried it in different browsers always with the same result. What am I missing?

...Please note the image is an ad for a snooker book. It is not my book nor am I trying to promote it in any way. It just happens to be the image that this behaviour first occurred with...

[pandy]

The rule, if there is one, must also target a certain file naming pattern. It's just the one image the OP doesn't see.

[Christian J]

Maybe the purpose is to prevent bots from downloading ads? "_ad" does sound advertizing related. OTOH "imgtest2.html" doesn't, and it still returns a 403 for me. So did the site's home page, BTW.