How valuable are the designs to most military parts though? If I am a government looking to buy cruise missiles (for example), do I really care if lots of people have the design? What I care about is whether a company will sell to me, how much it will cost to get the number I want made, whether they can continue to support them and, continue to produce them at the rate I want.

Sure, there are some technologies that I want to not be leaked to allies where significant parts of how they work aren't public knowledge (stealth material/coating composition maybe?) but, it is only those that I would actually care about being secret, if I was a government/military buying hardware. If it isn't hard to figure out, I'd rather it was easy to access the knowledge so my engineers/technicians can get access.

BAE Systems are a significant weapons manufacturer. Sorting security 'problems' like the one you are describing isn't that expensive so, if it was bothering their clients, you'd think they would do it. It is more plausible that they just don't care that the this information is so easy to access because it is easier either for them or their clients.