Need a LEGAL .iso of Windows 10 Pro - Pre Anniversary Edition for clean install.

Thanks for the clarifications!
The real, underlying problem I have is so strange and bizarre that I went all the way back to 14393 in hopes that it could never contain the same bugs or inconsistencies...

Unfortunately, it produced the same error as The Insider build - as well as the Anniversary. So, finding the earliest version of Windows 10 solved the problem I indicated in the post - which was getting the ISO itself, but It did not, however, resolve the real issue reported here:

https://techcommunity.microsoft.com/t5/Windows-10/Windows-Security-and-Enter-Network-Credentials-Pop-up-I-am/m-p/91415#M198

This little conundrum must be quite difficult because no one has even replied to it. Do you think that you could take a stab at it?

Thanks