That's odd. 077 115 441 is common to PF and FL V8s, and the part in the 077 198 405 kit fits fine. 405 continues in to the D3 where the belt-dive V8 was used and also fits the Touareg variant of the belt V8 so at least part of what you found would appear to be correct. I can only assume that the aftermarket version is manufactured incorrectly?
077 198 405 is still available from Audi (rather than Tradition) and should be so until ~2023 based on its last application (2008 D3) so hopefully you shouldn't have too much difficulty getting one. There is stock in the UK so Prestige should be able to order one and ship it out to you. If not, I keep them in stock too, but Prestige is obviously first call for parts
Worth noting is that I have a batch of aftermarket versions of this, die-cut from rubber sheet by an enterprising Polish D2 enthusiast. I've not tried one yet so can't vouch for them, but if you want to give one a go they're a bunch cheaper than the full kit. I also have dimensions written down somewhere of the o-ring for the water pipes if you need that - a generic one is pence compared to £4 each from Audi!