The crediting should be simple, and the rules IMO are allready quite clear of how you make them. Common sense says allready, when you don't add anything of your own work on some drawing, then it doesen't deserve crediting. We have always leaved the actuall limit up to each own conscience to do it, and if someone is not capable of making judgment over these things, then perhaps he should not be kitbashing SB stuff at first place. (Or contact the staff if you are not sure is also an option)
But we are not going to start make up detailed pixel-per-pixel guidelines where the borderlines go over crediting. I still urge everyone to use their own brains on it, and remember that gentleman is not drawing in purpose of making others feel bad (this goes also in the way, to those commenting the drawings)
When I am working with my own AUs, there have been cases when I've used existing real life ships that have been drewn by someone else. Usually in those cases the drawings have been drawn in so different style compared to my own, that even the work to bring them uniform for the rest of my work has always sufficed for crediting.
Sometimes when this has not been nesserical, Ive repainted and readjusted the drawings nevertheless to require proper crediting. In really few cases Ive only needed to change the flags and perhaps add an pennant number. Then I've not credited myself over it.
But then again, for me makin AUs is always about drawing ships anyways...
When the rules of the Board and Community is a resoundingly clear "Don't Redraw What Has Already Been Drawn", people who create alternate nations using real life ships... well, they ain't got a lot of options.
Actually, the spirit of this unrwritten rule is intended for the actuall bucket and its real-life section. AUs have always been the seccondary surpluss "playground" to the main work of us to use the real life ships for something ligther and casual purposes. I wont mind if someone wants to have some reallife ship on his AU projects, and wants to do it himself, despite the thing is allready in the bucket, to go drawn the ship or equipment itself renewly. We are not uploading AU stuff anyways, so there is no harm for duplications. In fact if I would in some theorethical level drawing "western" AUs, I would propably drawing all the equipmentry renew. This could be actually worth of considering to those who have this redrawing urge that have in past coused some unnesserical drama.