It looks more like a Birkin to me (except for the yummy HAC plates). Either way, it is so unique that I would keep it. Maybe it was a super VIP special order? I was told Janet Jackson recently ordered a Birkin with longer shoulder straps and they did it, so maybe it was specifically designed like this? It's really beautiful!
As far as your decision to keep, my 50cm Birkin with toile lining weighs 5-6lbs I just carried it packed to 16lbs total inc. bag weight(with a heavy laptop) and I felt I was at my limit(to shlep through 2 airports), and the bags limit. traveling domestic first class my carry on limit was 25lbs but stowing it in the bin uptop was interesting because I had to squish it in. I almost bought a HAC and wondered how a 'structured' bag would fit uptop. I like the looks of your bag. It looks like a custom hybred?
The handles look very similar. I think you would rather have the thicker and shorter handles associated on a regular travel HAC. I feel the same way too. The client might have special order it the way it is. If this really bothers you, I would return it. One thing about slimmer handles is they feel softer and more flexible.
Thanks everyone I have did some deep soul searching for lack of better words and I do love the bag I will just have to take whatever weird criticizism I get for having such an odd piece. Thanks again ladies and gents and now I just have motivation for traditional pieces